Lamba Wars answers the question of what happens when you build a real-time strategy game in one of the most celebrated, beloved game universes ever created.

The amazing mod puts a real-time strategy game within the world of Half-Life 2, pitting human Rebels against the alien Combine race in a battle for survival. After surviving the Greenlight process earlier this year, Lambda Wars was released on Steam last week. Because the game is a mod built in Valve's Source engine, it's completely free.

At its core, Lambda Wars follows conventional RTS wisdom: Build a base, gather resources, train an army, annihilate your opponent. But this is more than a Half-Life-themed skin slapped on another RTS engine. Technically Lambda Wars is a mod for Alien Swarm—a free-to-play top-down shooter Valve released in 2010—but that game's engine has been overhauled and made into a fresh real-time strategy game here. Resource gathering and building style will look familiar to players of the Warhammer 40,000: Dawn of War series, and the game won't be foreign to anyone with experience in the genre.

The two playable factions, the Rebels and the Combine, field armies comprised entirely of units and characters from the Half-Life universe. More than that, though, the game exudes the Half-Life feeling. Rebel troops take their orders from Dr. Gordon Freeman (you). Shoot down a lumbering alien and a headcrab will burst forth, leaping toward your units to infect a new victim.

But Lambda Wars is not without problems. I found many character models, at least on the Rebel side, too similar to tell apart. Zoomed in, each unit has distinct features, but at the standard aerial viewing distance used in these games, their humanoid figures were largely indistinguishable.

Worse, I found the game occasionally had problems with pathfinding—that is, when ordering a unit to move somewhere, it sometimes would get stymied when encountering objects in its path. I figure this is a side-effect of one of Lambda Wars' cooler features—a cover system not unlike X-COM: Enemy Unknown, where units can hug walls, barricades, and other objects to gain a defensive bonus in combat. The problem is that when sending a unit beyond an object it could take cover behind, it occasionally forgets to walk around the obstacle and instead hunkers down.

But those problems aside, Lambda Wars is a great way to get your RTS fix in the Half-Life universe. And it's certainly a quicker way to get your Half-Life fix than waiting for Half-Life 3.