SourceForts is a Half Life 2: Deathmatch mod where Combine and Rebels face off in capture-the-flag action with a big twist. Players build bases and other structures with blocks and plates in the "build phase" to protect themselves and their team's flag (which is actually a ball), and then fight it out and try to capture the enemy's flag in the "combat phase". Once the combat phase has ended, it's back to build, where players can repair, improve, and change their creations to ready them for the next combat phase.

source: Steam / Game Mods