Lambda Wars (formerly Half-Life 2: Wars) brings the struggle between the human resistance and their Combine overlords to a multiplayer real-time strategy setting. The Lambda Wars beta only requires Alien Swarm installed to play. It features 3 multiplayer game modes: Sandbox, Annihilation and Overrun, showcasing the Rebels and Combine playable factions and a full-fledged RTS interface and HUD. There is also a singleplayer tutorial and a handful of singleplayer missions.
Small portion of an unfinished action adventure mission. It has items you collect, units you train and other stuff you'd expect from the genre.
I think the multiplayer style of building an economy would be boring for singleplayer so, in this mission, you get resources by killing enemies and collecting items and your units instantly train.
At the end of each section there are new units and upgrades and a flag you capture. When you capture all the flags on the map, you advance to the next one. There are subordinate sections and other areas you can explore.