<s>
Sentinel	B-Operating_System
:	O
Mars	O
Defense	O
is	O
an	O
iOS	B-Application
sci-fi	O
tower	B-Application
defense	I-Application
game	I-Application
developed	O
by	O
British	O
indie	O
studio	O
Origin8	O
Technologies	O
and	O
released	O
on	O
February	O
27	O
,	O
2009	O
.	O
</s>
<s>
It	O
was	O
followed	O
by	O
the	O
sequels	O
Sentinel	B-Operating_System
2	O
:	O
Earth	O
Defense	O
(	O
2009	O
)	O
,	O
Sentinel	B-Operating_System
3	O
:	O
Homeworld	O
(	O
2010	O
)	O
,	O
and	O
Sentinel	B-Operating_System
4	O
:	O
Dark	O
Star	O
(	O
2014	O
)	O
.	O
</s>
<s>
148Apps	O
wrote	O
"	O
Sentinel	B-Operating_System
is	O
easily	O
one	O
of	O
the	O
best	O
tower	B-Application
defense	I-Application
games	I-Application
in	O
the	O
App	B-Application
Store	I-Application
,	O
and	O
genre	O
fans	O
would	O
be	O
crazy	O
not	O
to	O
get	O
this	O
one	O
.	O
</s>
<s>
Appsafari	O
said	O
"	O
Are	O
you	O
a	O
fan	O
of	O
TD	B-Application
games	I-Application
for	O
the	O
iPhone	B-Device
?	O
</s>
<s>
Sentinel	B-Operating_System
Mars	O
Defense	O
from	O
origin8	O
is	O
a	O
Tower	O
of	O
Defense	O
game	O
for	O
the	O
iPhone	B-Device
with	O
amazing	O
graphics	O
of	O
rendered	O
environments	O
,	O
towers	O
and	O
enemies	O
"	O
.	O
</s>
<s>
SlideToPlay	O
said	O
"	O
Sentinel	B-Operating_System
2	O
manages	O
to	O
stand	O
out	O
amidst	O
the	O
crowded	O
tower	B-Application
defense	I-Application
genre	O
"	O
TouchArcade	O
wrote	O
"	O
If	O
you	O
happen	O
to	O
be	O
new	O
to	O
the	O
tower	B-Application
defense	I-Application
genre	O
,	O
Sentinel	B-Operating_System
is	O
a	O
great	O
place	O
to	O
start	O
,	O
especially	O
with	O
the	O
current	O
price	O
of	O
99¢	O
.	O
</s>
<s>
Since	O
Sentinel	B-Operating_System
2	O
is	O
an	O
evolution	O
of	O
the	O
original	O
,	O
if	O
you	O
enjoy	O
the	O
first	O
,	O
you	O
will	O
enjoy	O
the	O
sequel	O
.	O
</s>
<s>
Both	O
are	O
among	O
my	O
favorite	O
games	O
,	O
and	O
while	O
there	O
is	O
no	O
shortage	O
of	O
tower	B-Application
defense	I-Application
games	I-Application
for	O
the	O
iPhone	B-Device
,	O
in	O
my	O
opinion	O
,	O
both	O
Sentinel	B-Operating_System
games	O
are	O
among	O
the	O
best	O
the	O
App	B-Application
Store	I-Application
has	O
to	O
offer.	O
"	O
</s>
<s>
AppSpy	O
said	O
"	O
The	O
game	O
is	O
n’t	O
bad	O
;	O
it	O
’s	O
just	O
not	O
one	O
of	O
the	O
better	O
tower	B-Application
defence	I-Application
games	O
and	O
may	O
not	O
be	O
worth	O
the	O
current	O
price.	O
"	O
</s>
<s>
148Apps	O
wrote	O
"	O
A	O
combination	O
of	O
fantastic	O
graphics	O
and	O
multi-layered	O
complex	O
gameplay	O
make	O
Sentinel	B-Operating_System
2	O
the	O
best	O
tower	B-Application
defense	I-Application
game	I-Application
on	O
the	O
App	B-Application
Store	I-Application
.	O
</s>
<s>
Be	O
warned	O
-	O
it	O
may	O
be	O
tough	O
to	O
go	O
back	O
to	O
seemingly	O
simple	O
TD	B-Application
games	I-Application
after	O
experiencing	O
what	O
Sentinel	B-Operating_System
2	O
has	O
to	O
offer.	O
"	O
</s>
<s>
Sentinel	B-Operating_System
3	O
has	O
a	O
Metacritic	O
score	O
of	O
87%	O
based	O
on	O
9	O
critic	O
ratings	O
.	O
</s>
<s>
Sentinel	B-Operating_System
4	O
has	O
a	O
Metacritic	O
score	O
of	O
72%	O
based	O
on	O
5	O
critic	O
reviews	O
.	O
</s>
