Base 2 Base is a Half-Life 2 Multiplayer Modification which puts 2 teams against each other (Red & Blue) and they have to fight to reach a teleport that is above the enemies base. When a player from that team goes through the teleport they are teleported to a safehouse where they can watch the battlefield on a monitor. First team to have all of their players through the teleport into the safehouse wins, and if a player is in the safehouse and wants to return to the battlefield they can do so by suiciding.
This is the Blue Team's turret as you can see its placed outside of their spawn for easy access. The colour of the turret is still blue so its easy for players to identify what team the turret is on.