I wonder, can we make the bridge to raise up (to let boat go through), or to release it back down so land units can go over the water by using a toggle button (similar to Wall Gate building in SVN (Soon to be alpha11?)? That way, when its raised up, the terrain that bridge was covering is now a water type. When bridge release it back down, the terrain is no longer a water (therefore boat can't pass it) but now land unit can walk over it.