Iron Brigade is considered to be a hybrid game, mixing the gameplay of tower defense games with third-person shooters and incorporating role-playing video game elements. Players control a "trench", a giant war machine that can move about the battlefield as needed. The player's goal is to protect their base, the ship U.S.S. McKinley from increasingly difficult waves of Monovisions. Though a player's trench may be disabled by enemy attacks, the player can spend time to repair the machine and continue on; the game is only lost when the base is overrun by the Monovisions. During the attacks, the player has the ability to ground its trench at a fixed spot, or move about the field to fire on foes.
