<s>
Alien	B-Application
Swarm	I-Application
is	O
a	O
multiplayer	O
top-down	O
shooter	O
video	O
game	O
developed	O
by	O
Valve	O
.	O
</s>
<s>
It	O
is	O
a	O
remake	O
of	O
a	O
mod	O
for	O
Unreal	B-Application
Tournament	I-Application
2004	I-Application
,	O
and	O
it	O
was	O
developed	O
by	O
the	O
original	O
team	O
,	O
who	O
were	O
hired	O
by	O
Valve	O
during	O
the	O
course	O
of	O
the	O
development	O
process	O
.	O
</s>
<s>
Alien	B-Application
Swarm	I-Application
is	O
a	O
top-down	O
shooter	O
,	O
where	O
four	O
players	O
can	O
join	O
a	O
single	O
co-operative	O
game	O
,	O
the	O
aim	O
of	O
which	O
is	O
to	O
progress	O
through	O
science	O
fiction-themed	O
levels	O
while	O
eliminating	O
waves	O
of	O
aliens	O
.	O
</s>
<s>
Alien	B-Application
Swarm	I-Application
was	O
originally	O
a	O
mod	O
for	O
Unreal	B-Application
Tournament	I-Application
2004	I-Application
.	O
</s>
<s>
A	O
Source	B-Application
engine	I-Application
sequel	O
to	O
the	O
original	O
Unreal	B-Application
Tournament	I-Application
2004	I-Application
mod	O
was	O
announced	O
in	O
2005	O
,	O
under	O
the	O
title	O
of	O
Alien	B-Application
Swarm	I-Application
:	O
Infested	O
.	O
</s>
<s>
In	O
July	O
2010	O
with	O
the	O
announcement	O
of	O
Alien	B-Application
Swarm	I-Application
,	O
it	O
was	O
revealed	O
that	O
Valve	O
had	O
hired	O
the	O
team	O
behind	O
Alien	B-Application
Swarm	I-Application
,	O
who	O
had	O
finished	O
the	O
mod	O
between	O
working	O
on	O
other	O
Valve	O
products	O
such	O
as	O
Left	B-Application
4	I-Application
Dead	I-Application
and	O
Portal	B-Application
2	I-Application
.	O
</s>
<s>
An	O
Alien	B-Application
Swarm	I-Application
software	B-Application
development	I-Application
kit	I-Application
(	O
SDK	O
)	O
including	O
buildable	B-Language
source	B-Application
code	O
was	O
released	O
alongside	O
the	O
game	O
.	O
</s>
<s>
It	O
is	O
free	O
to	O
all	O
users	O
of	O
Steam	B-Application
,	O
rather	O
than	O
only	O
to	O
owners	O
of	O
existing	O
Source	B-Application
games	O
(	O
as	O
is	O
the	O
case	O
with	O
the	O
'	O
mainline	O
 '	O
Source	B-Application
SDK	O
)	O
.	O
</s>
<s>
This	O
allows	O
total	O
conversion	O
mods	O
,	O
which	O
do	O
not	O
rely	O
on	O
content	O
from	O
other	O
Valve	O
games	O
,	O
to	O
be	O
free	O
to	O
all	O
as	O
well	O
-	O
a	O
significant	O
business	O
decision	O
that	O
echoes	O
the	O
strategy	O
of	O
the	O
Unreal	B-Application
Development	I-Application
Kit	I-Application
.	O
</s>
<s>
The	O
SDK	O
and	O
software	B-License
license	I-License
allows	O
arbitrary	O
usage	O
and	O
sharing	O
but	O
only	O
for	O
non-commercial	O
purposes	O
.	O
</s>
<s>
Alien	B-Application
Swarm	I-Application
received	O
"	O
generally	O
favorable	O
reviews	O
"	O
according	O
to	O
the	O
review	B-Application
aggregation	I-Application
website	O
Metacritic	O
.	O
</s>
