<s>
Tower	B-Application
defense	I-Application
(	O
TD	O
)	O
is	O
a	O
subgenre	O
of	O
strategy	B-Application
games	I-Application
where	O
the	O
goal	O
is	O
to	O
defend	O
a	O
player	O
's	O
territories	O
or	O
possessions	O
by	O
obstructing	O
the	O
enemy	O
attackers	O
or	O
by	O
stopping	O
enemies	O
from	O
reaching	O
the	O
exits	O
,	O
usually	O
achieved	O
by	O
placing	O
defensive	O
structures	O
on	O
or	O
along	O
their	O
path	O
of	O
attack	O
.	O
</s>
<s>
Tower	B-Application
defense	I-Application
is	O
seen	O
as	O
a	O
subgenre	O
of	O
real-time	O
strategy	B-Application
video	I-Application
games	I-Application
,	O
due	O
to	O
its	O
real-time	O
origins	O
,	O
even	O
though	O
many	O
modern	O
tower	B-Application
defense	I-Application
games	I-Application
include	O
aspects	O
of	O
turn-based	O
strategy	O
.	O
</s>
<s>
The	O
tower	B-Application
defense	I-Application
genre	O
can	O
trace	O
its	O
lineage	O
back	O
to	O
the	O
golden	O
age	O
of	O
arcade	O
video	O
games	O
in	O
the	O
1980s	O
.	O
</s>
<s>
The	O
object	O
of	O
the	O
arcade	O
game	O
Space	B-Application
Invaders	I-Application
,	O
released	O
in	O
1978	O
,	O
was	O
to	O
defend	O
the	O
player	O
's	O
territory	O
(	O
represented	O
by	O
the	O
bottom	O
of	O
the	O
screen	O
)	O
against	O
waves	O
of	O
incoming	O
enemies	O
.	O
</s>
<s>
The	O
1980	O
game	O
Missile	B-Application
Command	I-Application
changed	O
that	O
by	O
giving	O
shields	O
a	O
more	O
strategic	O
role	O
.	O
</s>
<s>
Missile	B-Application
Command	I-Application
was	O
also	O
the	O
first	O
of	O
its	O
kind	O
to	O
make	O
use	O
of	O
a	O
pointing	O
device	O
,	O
a	O
trackball	B-Device
,	O
enabling	O
players	O
to	O
use	O
a	O
crosshair	O
.	O
</s>
<s>
The	O
innovation	O
was	O
ahead	O
of	O
its	O
time	O
and	O
anticipated	O
the	O
genre	O
's	O
later	O
boom	O
,	O
which	O
was	O
paved	O
by	O
the	O
wide	O
adoption	O
of	O
the	O
computer	B-Device
mouse	I-Device
.	O
</s>
<s>
Additionally	O
,	O
in	O
Missile	B-Application
Command	I-Application
,	O
the	O
sole	O
target	O
of	O
the	O
attackers	O
is	O
the	O
base	O
,	O
not	O
a	O
specific	O
player	O
character	O
.	O
</s>
<s>
While	O
later	O
arcade	O
games	O
like	O
Defender	B-Application
(	O
1981	O
)	O
and	O
Choplifter	B-Application
(	O
1982	O
)	O
lacked	O
the	O
strategy	O
element	O
of	O
Missile	B-Application
Command	I-Application
,	O
they	O
began	O
a	O
trend	O
of	O
games	O
that	O
shifted	O
the	O
primary	O
objective	O
to	O
defending	O
non-player	O
items	O
.	O
</s>
<s>
Parker	O
Brothers	O
 '	O
1982	O
title	O
Star	B-Application
Wars	I-Application
:	I-Application
The	I-Application
Empire	I-Application
Strikes	I-Application
Back	I-Application
for	O
the	O
Atari	B-General_Concept
2600	I-General_Concept
was	O
one	O
of	O
the	O
first	O
tie-ins	O
to	O
popularize	O
the	O
base	O
defense	O
style	O
.	O
</s>
<s>
Sorcerer	O
's	O
Apprentice	O
for	O
the	O
Atari	B-General_Concept
2600	I-General_Concept
featured	O
Mickey	B-Application
Mouse	I-Application
and	O
was	O
first	O
published	O
in	O
1983	O
.	O
</s>
<s>
Nintendo	O
's	O
popular	O
1980s	O
Game	B-Device
&	I-Device
Watch	I-Device
handheld	B-Application
games	I-Application
featured	O
many	O
popular	O
precursors	O
.	O
</s>
<s>
The	O
later	O
titles	O
utilized	O
multiple	O
articulating	B-Device
screens	I-Device
to	O
increase	O
the	O
difficulty	O
for	O
players	O
.	O
</s>
<s>
Despite	O
the	O
early	O
rush	O
of	O
archetypal	O
titles	O
,	O
ultimately	O
there	O
was	O
a	O
general	O
decline	O
in	O
fixed-cell	O
games	O
,	O
due	O
to	O
their	O
technical	O
limitations	O
,	O
simplistic	O
gameplay	O
,	O
and	O
the	O
rise	O
of	O
personal	O
computers	O
and	O
handhelds	B-Application
the	O
Game	B-Device
Boy	I-Device
;	O
correspondingly	O
,	O
this	O
genre	O
also	O
declined	O
.	O
</s>
<s>
Early	O
PC	O
gaming	O
examples	O
include	O
the	O
1984	O
Commodore	O
64	O
titles	O
Gandalf	O
the	O
Sorcerer	O
,	O
a	O
shooter	O
with	O
tower	B-Application
defense	I-Application
elements	O
,	O
and	O
Imagine	O
Software	O
's	O
1984	O
release	O
Pedro	B-Application
.	O
</s>
<s>
Pedro	B-Application
,	O
a	O
garden	O
defense	O
game	O
,	O
introduced	O
new	O
gameplay	O
elements	O
,	O
including	O
different	O
enemy	O
types	O
as	O
well	O
as	O
the	O
ability	O
to	O
place	O
fixed	O
obstructions	O
,	O
and	O
to	O
build	O
and	O
repair	O
the	O
player	O
's	O
territory	O
.	O
</s>
<s>
Rampart	B-Application
,	O
released	O
in	O
1990	O
,	O
is	O
generally	O
considered	O
to	O
have	O
established	O
the	O
prototypical	O
tower	B-Application
defense	I-Application
.	O
</s>
<s>
Rampart	B-Application
introduced	O
player	O
placed	O
defenses	O
that	O
automatically	O
attack	O
incoming	O
enemies	O
.	O
</s>
<s>
While	O
Rampart	B-Application
was	O
popular	O
,	O
similar	O
games	O
were	O
rarely	O
seen	O
until	O
the	O
widespread	O
adoption	O
of	O
the	O
computer	B-Device
mouse	I-Device
on	O
the	O
PC	O
.	O
</s>
<s>
Tower	B-Application
defense	I-Application
gameplay	O
also	O
made	O
an	O
appearance	O
on	O
consoles	O
with	O
several	O
minigames	O
in	O
the	O
Final	B-Application
Fantasy	I-Application
series	O
,	O
including	O
a	O
tower-defense	O
minigame	O
in	O
Final	B-Application
Fantasy	I-Application
VI	I-Application
(	O
1994	O
)	O
and	O
the	O
Fort	O
Condor	O
minigame	O
in	O
Final	B-Application
Fantasy	I-Application
VII	I-Application
(	O
1997	O
)	O
,	O
which	O
was	O
also	O
one	O
of	O
the	O
first	O
to	O
feature	O
3D	O
graphics	O
.	O
</s>
<s>
Attack	B-Application
of	I-Application
the	I-Application
Mutant	I-Application
Penguins	I-Application
for	O
the	O
Atari	O
Jaguar	O
and	O
MS-DOS	O
was	O
released	O
in	O
1995	O
.	O
</s>
<s>
Dungeon	B-Application
Keeper	I-Application
(	O
1997	O
)	O
had	O
players	O
defend	O
the	O
Dungeon	O
Heart	O
,	O
a	O
gigantic	O
gem	O
at	O
the	O
centre	O
of	O
your	O
dungeon	O
,	O
which	O
,	O
if	O
destroyed	O
,	O
would	O
cause	O
the	O
player	O
to	O
lose	O
the	O
game	O
.	O
</s>
<s>
A	O
central	O
theme	O
of	O
the	O
3D	O
first	O
person	O
shooter	O
Turok	B-Application
2	I-Application
:	I-Application
Seeds	I-Application
of	I-Application
Evil	I-Application
(	O
1998	O
)	O
was	O
to	O
defend	O
Energy	O
Totems	O
against	O
hordes	O
of	O
attackers	O
.	O
</s>
<s>
Fortress	B-Application
was	O
released	O
for	O
the	O
Game	B-Device
Boy	I-Device
Advance	I-Device
in	O
2001	O
.	O
</s>
<s>
As	O
real-time	O
strategy	B-Application
games	I-Application
gained	O
popularity	O
in	O
PC	O
gaming	O
,	O
many	O
introduced	O
tower	B-Application
defense	I-Application
modes	O
in	O
their	O
gameplay	O
,	O
particularly	O
in	O
multiplayer	O
.	O
</s>
<s>
In	O
February	O
2006	O
,	O
the	O
custom	O
maps	O
for	O
Warcraft	B-Application
III	I-Application
(	O
2002	O
)	O
,	O
Element	O
TD	O
and	O
Gem	B-Application
Tower	I-Application
Defense	I-Application
,	O
which	O
were	O
initially	O
created	O
in	O
Warcraft	B-Application
III	I-Application
World	O
Editor	O
,	O
almost	O
single-handedly	O
rekindled	O
the	O
genre	O
.	O
</s>
<s>
Between	O
2007	O
and	O
2008	O
,	O
the	O
genre	O
became	O
a	O
phenomenon	O
,	O
due	O
in	O
part	O
to	O
the	O
popularity	O
of	O
the	O
tower	B-Application
defense	I-Application
mode	O
in	O
real	O
time	O
strategy	B-Application
games	I-Application
,	O
but	O
mainly	O
due	O
to	O
the	O
rise	O
of	O
Adobe	B-Operating_System
Flash	I-Operating_System
independent	O
developers	O
as	O
well	O
as	O
the	O
emergence	O
of	O
major	O
smartphone	B-Application
app	B-Application
stores	I-Application
from	O
Apple	O
and	O
Google	O
.	O
</s>
<s>
The	O
first	O
stand-alone	O
browser	B-Protocol
games	I-Protocol
emerged	O
in	O
2007	O
.	O
</s>
<s>
Among	O
them	O
were	O
the	O
extremely	O
popular	O
titles	O
Flash	B-Operating_System
Element	I-Operating_System
Tower	I-Operating_System
Defense	I-Operating_System
released	O
in	O
January	O
,	O
Desktop	B-Operating_System
Tower	I-Operating_System
Defense	I-Operating_System
released	O
in	O
March	O
and	O
Antbuster	O
released	O
in	O
May	O
.	O
</s>
<s>
Desktop	B-Operating_System
Tower	I-Operating_System
Defense	I-Operating_System
earned	O
an	O
Independent	O
Games	O
Festival	O
award	O
,	O
and	O
its	O
success	O
led	O
to	O
a	O
version	O
created	O
for	O
the	O
mobile	O
phone	O
by	O
a	O
different	O
developer	O
.	O
</s>
<s>
Another	O
significant	O
Flash	O
title	O
released	O
in	O
2008	O
was	O
GemCraft	B-Application
.	O
</s>
<s>
Handheld	B-Application
game	I-Application
console	I-Application
were	O
not	O
ignored	O
in	O
the	O
boom	O
and	O
titles	O
included	O
Lock	B-Application
's	I-Application
Quest	I-Application
and	O
Ninjatown	B-Device
released	O
in	O
September	O
and	O
October	O
respectively	O
.	O
</s>
<s>
Bloons	B-Device
Tower	I-Device
Defense	I-Device
was	O
first	O
published	O
in	O
2007	O
,	O
one	O
of	O
many	O
in	O
a	O
series	O
of	O
balloon	O
themed	O
multi-platform	O
games	O
released	O
.	O
</s>
<s>
The	O
genre	O
's	O
success	O
also	O
led	O
to	O
new	O
releases	O
on	O
PC	O
and	O
video	B-Device
game	I-Device
consoles	I-Device
.	O
</s>
<s>
Popular	O
2008	O
titles	O
included	O
PixelJunk	B-Application
Monsters	I-Application
released	O
in	O
January	O
,	O
Defense	B-Application
Grid	I-Application
:	I-Application
The	I-Application
Awakening	I-Application
and	O
Savage	B-Device
Moon	I-Device
in	O
December	O
.	O
</s>
<s>
Also	O
released	O
in	O
2008	O
were	O
geoDefense	B-Application
Swarm	I-Application
,	O
geoDefense	B-Application
,	O
GemCraft	B-Application
,	O
Fieldrunners	B-Device
,	O
Harvest	B-Application
:	I-Application
Massive	I-Application
Encounter	I-Application
and	O
Crystal	B-Application
Defenders	I-Application
.	O
</s>
<s>
GauntNet	B-Language
was	O
released	O
in	O
April	O
2009	O
.	O
</s>
<s>
Plants	B-Application
vs.	I-Application
Zombies	I-Application
released	O
in	O
May	O
2009	O
was	O
another	O
highly	O
popular	O
tower	B-Application
defense	I-Application
which	O
became	O
a	O
successful	O
series	O
on	O
mobile	O
devices	O
.	O
</s>
<s>
Also	O
released	O
that	O
year	O
were	O
Sentinel	B-Operating_System
,	O
TowerMadness	B-Application
,	O
Babel	B-Application
Rising	I-Application
,	O
Creeper	B-Application
World	I-Application
,	O
Sol	B-Application
Survivor	I-Application
,	O
Comet	B-Device
Crash	I-Device
,	O
Final	B-Application
Fantasy	I-Application
Crystal	I-Application
Chronicles	I-Application
:	I-Application
My	I-Application
Life	I-Application
as	I-Application
a	I-Application
Darklord	I-Application
,	O
South	O
Park	O
Let	O
's	O
Go	O
Tower	O
Defense	O
Play	O
!,	O
Starship	B-Device
Patrol	I-Device
and	O
Trenches	B-Application
.	O
</s>
<s>
With	O
the	O
arrival	O
of	O
Apple	B-Application
's	I-Application
App	I-Application
Store	I-Application
tower	B-Application
defense	I-Application
developers	O
adapted	O
quickly	O
to	O
the	O
touchscreen	O
interface	O
and	O
the	O
titles	O
were	O
among	O
the	O
most	O
downloaded	O
,	O
many	O
of	O
them	O
ported	O
directly	O
from	O
Flash	O
.	O
</s>
<s>
Among	O
the	O
more	O
notable	O
include	O
Kingdom	B-Application
Rush	I-Application
,	O
first	O
released	O
in	O
2011	O
,	O
which	O
sold	O
more	O
than	O
seventeen	O
million	O
copies	O
both	O
on	O
App	B-Application
store	I-Application
and	O
Play	B-Application
store	I-Application
.	O
</s>
<s>
By	O
the	O
end	O
of	O
the	O
boom	O
,	O
most	O
tower	B-Application
defense	I-Application
games	I-Application
were	O
still	O
using	O
side	O
scrolling	O
,	O
isometric	O
,	O
or	O
top-down	O
perspective	O
graphics	O
.	O
</s>
<s>
Iron	B-Application
Grip	I-Application
:	I-Application
Warlord	I-Application
,	O
released	O
in	O
November	O
,	O
2008	O
unsuccessfully	O
pioneered	O
the	O
first	O
person	O
perspective	O
shooter	O
with	O
the	O
genre	O
.	O
</s>
<s>
The	O
awkward	O
combination	O
of	O
experimental	O
tower	B-Application
defense	I-Application
mechanics	O
with	O
3D	O
graphics	O
was	O
not	O
well	O
received	O
,	O
but	O
later	O
titles	O
refined	O
its	O
execution	O
paving	O
the	O
way	O
for	O
a	O
popular	O
new	O
breed	O
of	O
games	O
.	O
</s>
<s>
Dungeon	B-Application
Defenders	I-Application
,	O
released	O
in	O
October	O
2010	O
,	O
was	O
one	O
of	O
the	O
first	O
tower	B-Application
defense	I-Application
games	I-Application
to	O
bring	O
the	O
genre	O
to	O
the	O
third	O
person	O
perspective	O
.	O
</s>
<s>
2010	O
saw	O
the	O
release	O
of	O
SteamWorld	B-Device
Tower	I-Device
Defense	I-Device
,	O
Protect	B-Application
Me	I-Application
Knight	I-Application
,	O
The	B-Device
Tales	I-Device
of	I-Device
Bearsworth	I-Device
Manor	I-Device
,	O
Revenge	B-Language
of	I-Language
the	I-Language
Titans	I-Language
,	O
Arrow	B-Device
of	I-Device
Laputa	I-Device
,	O
Toy	B-Application
Soldiers	I-Application
and	O
Robocalypse	B-Application
:	I-Application
Beaver	I-Application
Defense	I-Application
.	O
</s>
<s>
The	O
2011	O
title	O
Sanctum	B-Application
,	O
and	O
its	B-Application
2013	I-Application
sequel	I-Application
popularized	O
the	O
first	O
person	O
shooter	O
hybrid	O
that	O
was	O
pioneered	O
by	O
these	O
earlier	O
games	O
.	O
</s>
<s>
Orcs	B-Application
Must	I-Application
Die	I-Application
!	I-Application
</s>
<s>
Anomaly	B-Application
:	I-Application
Warzone	I-Application
Earth	I-Application
released	O
in	O
2011	O
introduced	O
a	O
variation	O
of	O
gameplay	O
which	O
has	O
been	O
described	O
as	O
"	O
reverse	O
tower	B-Application
defense	I-Application
"	O
,	O
"	O
tower	O
attack	O
"	O
,	O
and	O
"	O
tower	O
offense	O
"	O
.	O
</s>
<s>
Sequels	O
and	O
other	O
games	O
have	O
since	O
experimented	O
further	O
with	O
both	O
styles	O
of	O
tower	B-Application
defense	I-Application
.	O
</s>
<s>
Tiny	B-Application
Heroes	I-Application
,	O
Army	B-Application
of	I-Application
Darkness	I-Application
:	I-Application
Defense	I-Application
,	O
Iron	B-Application
Brigade	I-Application
,	O
Rock	B-Application
of	I-Application
Ages	I-Application
and	O
Trenches	B-Operating_System
2	I-Operating_System
were	O
also	O
released	O
in	O
2011	O
.	O
</s>
<s>
Defender	B-Application
's	I-Application
Quest	I-Application
,	O
Bad	B-Application
Hotel	I-Application
,	O
Toy	B-Application
Defense	I-Application
,	O
Strikefleet	B-Application
Omega	I-Application
,	O
Unstoppable	B-Application
Gorg	I-Application
,	O
Defenders	B-Application
of	I-Application
Ardania	I-Application
,	O
Orcs	B-Application
Must	I-Application
Die	I-Application
!	I-Application
</s>
<s>
2	O
,	O
Fieldrunners	B-Application
2	I-Application
,	O
Dillon	B-Application
's	I-Application
Rolling	I-Application
Western	I-Application
,	O
Oil	B-Device
Rush	I-Device
and	O
Elf	B-Application
Defense	I-Application
Eng	I-Application
all	O
came	O
out	O
in	O
2012	O
.	O
</s>
<s>
Around	O
this	O
period	O
the	O
genre	O
matured	O
,	O
gaining	O
recognition	O
as	O
a	O
distinct	O
sub-genre	O
of	O
strategy	B-Application
games	I-Application
and	O
returning	O
in	O
numerous	O
upgraded	O
versions	O
.	O
</s>
<s>
Chain	B-Application
Chronicle	I-Application
and	O
CastleStorm	B-Application
were	O
released	O
in	O
2013	O
.	O
</s>
<s>
Plants	B-Application
vs.	I-Application
Zombies	I-Application
2	I-Application
came	O
out	O
and	O
Prime	B-Application
World	I-Application
:	I-Application
Defenders	I-Application
featured	O
deck-building	O
mechanics	O
.	O
</s>
<s>
2014	O
saw	O
a	O
number	O
of	O
brand	O
new	O
titles	O
including	O
Space	B-Application
Run	I-Application
,	O
Dungeon	B-Application
of	I-Application
the	I-Application
Endless	I-Application
,	O
Island	B-Application
Days	I-Application
,	O
Final	B-Application
Horizon	I-Application
and	O
The	B-Application
Battle	I-Application
Cats	I-Application
as	O
well	O
the	O
return	O
of	O
Age	B-Application
of	I-Application
Empires	I-Application
:	I-Application
Castle	I-Application
Siege	I-Application
,	O
Defense	B-Application
Grid	I-Application
2	I-Application
and	O
TowerMadness	B-Application
2	I-Application
.	O
</s>
<s>
Deathtrap	B-Application
and	O
Krinkle	B-Device
Krusher	I-Device
were	O
first	O
published	O
in	O
2015	O
.	O
</s>
<s>
More	O
recent	O
titles	O
in	O
the	O
genre	O
include	O
Rock	B-Application
of	I-Application
Ages	I-Application
2	I-Application
:	I-Application
Bigger	I-Application
&	I-Application
Boulder	I-Application
(	O
2017	O
)	O
and	O
Orcs	B-Application
Must	I-Application
Die	I-Application
!	I-Application
</s>
<s>
Unchained	O
(	O
2017	O
)	O
,	O
Dillon	B-Device
's	I-Device
Dead-Heat	I-Device
Breakers	I-Device
(	O
2018	O
)	O
,	O
Eden	B-Application
Rising	I-Application
:	I-Application
Supremacy	I-Application
(	O
2018	O
)	O
,	O
Aegis	B-Application
Defenders	I-Application
(	O
2018	O
)	O
,	O
Bloons	B-Application
TD	I-Application
6	I-Application
(	O
2018	O
)	O
,	O
Arknights	B-Application
(	O
2019	O
)	O
,	O
Taur	B-Application
(	O
2020	O
)	O
,	O
Element	O
TD	O
2	O
(	O
2020	O
)	O
,	O
Buster	O
's	O
Tower	B-Application
Defense	I-Application
(	O
2021	O
)	O
,	O
and	O
Path	O
to	O
Nowhere	O
(	O
2022	O
)	O
.	O
</s>
<s>
With	O
the	O
advent	O
of	O
social	O
networking	O
service	O
applications	O
,	O
such	O
as	O
the	O
Facebook	O
Platform	O
,	O
tower	B-Application
defense	I-Application
has	O
become	O
a	O
popular	O
genre	O
with	O
titles	O
such	O
as	O
Bloons	B-Device
TD	I-Device
and	O
Plants	B-Application
vs.	I-Application
Zombies	I-Application
Adventures	O
making	O
the	O
transition	O
to	O
turn-based	O
play	O
.	O
</s>
<s>
Recent	O
releases	O
include	O
Star	B-Device
Fox	I-Device
Guard	I-Device
and	O
McDroid	B-Application
which	O
came	O
out	O
in	O
2016	O
.	O
</s>
<s>
On	O
the	O
platform	O
Roblox	B-Protocol
,	O
many	O
Tower	B-Application
Defense	I-Application
games	I-Application
have	O
been	O
created	O
,	O
the	O
most	O
notable	O
being	O
Tower	B-Application
Defense	I-Application
Simulator	O
(	O
2019	O
)	O
as	O
well	O
as	O
the	O
first	O
famous	O
one	O
being	O
Tower	O
Battles	O
(	O
2017	O
)	O
.	O
</s>
<s>
The	O
basic	O
gameplay	O
elements	O
of	O
tower	B-Application
defense	I-Application
are	O
:	O
</s>
<s>
What	O
distinguishes	O
tower	B-Application
defense	I-Application
base	O
defending	O
games	O
from	O
other	O
base	O
defending	O
games	O
(	O
such	O
as	O
Space	B-Application
Invaders	I-Application
,	O
or	O
other	O
games	O
where	O
bases	O
are	O
defended	O
)	O
is	O
the	O
player	O
's	O
ability	O
to	O
strategically	O
place	O
,	O
construct	O
or	O
summon	O
obstructive	O
constructions	O
and	O
constructive	O
obstructions	O
in	O
the	O
path	O
of	O
attacking	O
enemies	O
.	O
</s>
<s>
In	O
a	O
tower	B-Application
defense	I-Application
,	O
the	O
player	O
's	O
main	O
character	O
is	O
often	O
invincible	O
,	O
as	O
their	O
primary	O
objective	O
is	O
the	O
survival	O
of	O
the	O
base	O
rather	O
than	O
the	O
player	O
.	O
</s>
<s>
Some	O
features	O
of	O
modern	O
tower	B-Application
defense	I-Application
:	O
</s>
<s>
Many	O
modern	O
tower	B-Application
defense	I-Application
games	I-Application
evolved	O
from	O
real-time	O
to	O
turn-based	O
,	O
cycling	O
through	O
distinct	O
gameplay	O
phases	O
such	O
as	O
build	O
,	O
defend	O
,	O
repair	O
,	O
and	O
celebrate	O
.	O
</s>
<s>
Many	O
games	O
,	O
such	O
as	O
Flash	B-Operating_System
Element	I-Operating_System
Tower	I-Operating_System
Defense	I-Operating_System
feature	O
enemies	O
that	O
scamper	O
through	O
a	O
"	O
maze	O
"	O
,	O
which	O
allows	O
the	O
player	O
to	O
strategically	O
place	O
"	O
towers	O
"	O
for	O
optimal	O
effectiveness	O
.	O
</s>
<s>
However	O
,	O
some	O
versions	O
of	O
the	O
genre	O
force	O
the	O
user	O
to	O
create	O
the	O
"	O
maze	O
"	O
out	O
of	O
their	O
own	O
"	O
towers	O
"	O
,	O
such	O
as	O
Desktop	B-Operating_System
Tower	I-Operating_System
Defense	I-Operating_System
.	O
</s>
<s>
A	O
common	O
theme	O
in	O
tower	B-Application
defense	I-Application
games	I-Application
to	O
have	O
occasional	O
"	O
air	O
"	O
units	O
which	O
ignore	O
the	O
layout	O
of	O
the	O
board	O
(	O
i.e.	O
</s>
<s>
Some	O
tower	B-Application
defense	I-Application
games	I-Application
or	O
custom	O
maps	O
also	O
require	O
the	O
player	O
to	O
not	O
only	O
defend	O
their	O
own	O
board	O
but	O
send	O
out	O
enemies	O
to	O
attack	O
their	O
opponents	O
 '	O
game	O
boards	O
(	O
or	O
opponent-controlled	O
areas	O
of	O
a	O
common	O
game	O
board	O
)	O
in	O
return	O
.	O
</s>
<s>
Such	O
games	O
are	O
also	O
known	O
as	O
tower	B-Application
wars	I-Application
game	O
boards	O
.	O
</s>
<s>
On	O
June	O
3	O
,	O
2008	O
,	O
COM2US	O
Corporation	O
was	O
awarded	O
the	O
trademark	O
for	O
the	O
term	O
"	O
Tower	B-Application
Defense	I-Application
"	O
,	O
filed	O
on	O
June	O
13	O
,	O
2007	O
–	O
serial	O
number	O
3442002	O
.	O
</s>
<s>
The	O
corporation	O
is	O
reported	O
to	O
have	O
started	O
enforcing	O
the	O
trademark	O
:	O
in	O
early	O
2010	O
,	O
developers	O
of	O
games	O
on	O
Apple	B-Application
's	I-Application
App	I-Application
Store	I-Application
reported	O
receiving	O
messages	O
requiring	O
name	O
changes	O
for	O
their	O
games	O
,	O
citing	O
trademark	O
violation	O
.	O
</s>
<s>
Adding	O
the	O
phrase	O
"	O
Tower	B-Application
Defense	I-Application
"	O
(	O
in	O
capital	O
letters	O
)	O
to	O
the	O
description	O
of	O
an	O
app	O
submission	O
to	O
iTunesConnect	O
and	O
the	O
app	B-Application
store	I-Application
automatically	O
triggers	O
a	O
warning	O
that	O
the	O
submission	O
is	O
likely	O
to	O
be	O
rejected	O
for	O
use	O
of	O
the	O
term	O
;	O
however	O
,	O
writing	O
the	O
phrase	O
in	O
lower	O
case	O
is	O
still	O
acceptable	O
as	O
"	O
tower	B-Application
defense	I-Application
"	O
is	O
a	O
valid	O
description	O
of	O
a	O
game	O
style	O
.	O
</s>
