Unreal Software had developed Counter-Strike 2D, a “top down” version of Valve’s original game. And it’s FREE.
https://www.cs2d.com/index.php
Top down games don’t really interest me, but for the few die-hard CS fans, this may be of some value.
Only 7MB to download. 
Its been around for a while…
Makes me wonder what an Alien Swarm style CS game would be like…
Alien Swarm style CS game is too good. While playing this game as we cross the levels, then it becomes more interesting. The most interesting thing which I like the most in this game is 3d compositing, Amazing…!!
Founded in 2004, Leakfree.org became one of the first online communities dedicated to Valve’s Source engine development. It is more famously known for the formation of Black Mesa: Source under the 'Leakfree Modification Team' handle in September 2004.