<s>
Narbacular	B-Application
Drop	I-Application
is	O
a	O
2005	O
puzzle-platform	O
game	O
developed	O
by	O
Nuclear	O
Monkey	O
Software	O
.	O
</s>
<s>
The	O
gameplay	O
consists	O
of	O
navigating	O
a	O
dungeon	O
using	O
an	O
innovative	O
portal	B-Application
system	O
.	O
</s>
<s>
The	O
player	O
controls	O
two	O
interconnected	O
portals	B-Application
that	O
can	O
be	O
placed	O
on	O
any	O
non-metallic	O
surface	O
(	O
wall	O
,	O
ceiling	O
,	O
or	O
floor	O
)	O
.	O
</s>
<s>
The	O
developers	O
went	O
on	O
to	O
write	O
the	O
critically	O
acclaimed	O
Portal	B-Application
using	O
many	O
of	O
the	O
same	O
concepts	O
.	O
</s>
<s>
While	O
Narbacular	B-Application
Drop	I-Application
features	O
a	O
3D	O
world	O
reminiscent	O
of	O
such	O
first-person	O
shooters	O
as	O
Quake	B-Application
,	O
the	O
unique	O
portal	B-Application
element	O
and	O
the	O
character	O
's	O
lack	O
of	O
a	O
jump	O
ability	O
makes	O
navigation	O
and	O
puzzle-solving	O
very	O
unconventional	O
.	O
</s>
<s>
The	O
player	O
can	O
open	O
a	O
single	O
pair	O
of	O
interconnected	O
portals	B-Application
at	O
a	O
time	O
,	O
each	O
styled	O
as	O
a	O
huge	O
face	O
with	O
flaming	O
eyes	O
(	O
orange	O
or	O
blue	O
to	O
tell	O
them	O
apart	O
as	O
the	O
player	O
repositions	O
one	O
or	O
the	O
other	O
)	O
and	O
an	O
open	O
mouth	O
big	O
enough	O
to	O
see	O
and	O
walk	O
through	O
.	O
</s>
<s>
Positioned	O
with	O
a	O
point-and-click	O
interface	O
controlled	O
by	O
the	O
mouse	O
,	O
portals	B-Application
are	O
allowed	O
only	O
on	O
natural	O
surfaces	O
and	O
are	O
prohibited	O
from	O
any	O
metal	O
,	O
lava	O
or	O
other	O
artificial	O
surfaces	O
in	O
the	O
game	O
.	O
</s>
<s>
Aside	O
from	O
the	O
portals	B-Application
,	O
important	O
game	O
elements	O
include	O
switches	O
,	O
boxes	O
,	O
huge	O
rolling	O
boulders	O
which	O
can	O
crush	O
the	O
character	O
,	O
and	O
"	O
lava	O
turtles	O
"	O
that	O
help	O
the	O
player	O
traverse	O
stretches	O
of	O
magma	O
.	O
</s>
<s>
However	O
,	O
members	O
of	O
the	O
Narbacular	B-Application
Drop	I-Application
forum	O
community	O
have	O
created	O
a	O
catalog	O
of	O
custom	O
maps	O
.	O
</s>
<s>
Captured	O
by	O
a	O
demon	O
,	O
the	O
imprisoned	O
princess	O
discovers	O
that	O
the	O
dungeon	O
she	O
is	O
held	O
in	O
is	O
actually	O
a	O
sentient	O
elemental	B-Device
creature	O
named	O
Wally	O
.	O
</s>
<s>
Using	O
Wally	O
's	O
portal-making	O
ability	O
,	O
the	O
princess	O
sets	O
out	O
to	O
escape	O
and	O
defeat	O
the	O
demon	O
.	O
</s>
<s>
Valve	O
,	O
developers	O
of	O
the	O
Half-Life	O
series	O
,	O
became	O
interested	O
in	O
Narbacular	B-Application
Drop	I-Application
after	O
seeing	O
the	O
game	O
at	O
DigiPen	O
's	O
annual	O
career	O
fair	O
.	O
</s>
<s>
The	O
team	O
developed	O
Portal	B-Application
in	O
approximately	O
two	O
years	O
and	O
four	O
months	O
after	O
joining	O
Valve	O
.	O
</s>
<s>
In	O
Portal	B-Application
,	O
the	O
player-controlled	O
silent	O
protagonist	O
,	O
Chell	O
,	O
is	O
challenged	O
and	O
taunted	O
by	O
an	O
artificial	B-Application
intelligence	I-Application
named	O
GLaDOS	O
to	O
complete	O
each	O
puzzle	O
in	O
the	O
Aperture	O
Science	O
Enrichment	O
Center	O
.	O
</s>
<s>
The	O
player	O
must	O
solve	O
puzzles	O
using	O
"	O
the	O
Aperture	O
Science	O
Handheld	O
Portal	B-Application
Device	O
"	O
,	O
a	O
device	O
that	O
can	O
create	O
portals	B-Application
between	O
two	O
flat	O
planes	O
.	O
</s>
<s>
Certain	O
elements	O
have	O
been	O
retained	O
from	O
Narbacular	B-Application
Drop	I-Application
,	O
such	O
as	O
the	O
system	O
of	O
identifying	O
the	O
two	O
unique	O
portal	B-Application
endpoints	O
with	O
the	O
colors	O
orange	O
and	O
blue	O
.	O
</s>
<s>
Portal	B-Application
was	O
released	O
on	O
October	O
10	O
,	O
2007	O
on	O
PC	O
,	O
Xbox	B-Operating_System
360	I-Operating_System
and	O
PlayStation	B-Operating_System
3	I-Operating_System
,	O
as	O
part	O
of	O
The	B-Application
Orange	I-Application
Box	I-Application
,	O
to	O
critical	O
and	O
commercial	O
success	O
.	O
</s>
<s>
A	O
sequel	O
,	O
Portal	B-Application
2	I-Application
,	O
was	O
released	O
on	O
April	O
19	O
,	O
2011	O
.	O
</s>
