Contra is a freeware modification for Command and Conquer Generals: Zero Hour real-time strategy game and one of the first mods for this game. It adds many new units as well as numerous new upgrades, new general's powers and buildings. It also adds new sounds, maps, bug fixes, enhanced graphics, and other effects, as well as three new generals.
Running some test...
wait...is that a sentry turret upgrade?
Yes, you noticed. Security systems are back, now only as machine gun which is effective vs infantry and light tanks. Systems are balanced and useful.