Castles mode

Add a game mode where players can build their own castles, with player-placed walls, with the goal of defending the reactor and the overmind, which are statically placed and can not be moved, can not be repaired, but have a lot of health. The game should also ensure that there is at least a set number of paths to the key structures which are not obstructed by walls. Alternatively, such paths can be specified in advance by the mapper.