Here's something that I think is in the spirit of the game and has a good chance of working:
Frame the game as two armies racing to occupy an oasis (four center squares of the board) in the desert. Consider that oasis occupied if either the enemy's king is captured, or if your king can camp there for X moves (if your king can camp there, the area can't be part of the front anymore, so it is truly yours)
I chose an oasis rather than a true stronghold such as a fort or castle because a fort or castle would have to mean the introduction of separate rules for attacks into and out of the stronghold (hm, that might be fun, too. Consider a case where captures into the four center squares require the capturer to have at least two means to make that capture.)
