My whole thinking is momentum.
Basically if you capture a fort, your army keeps moving on while your new fort stays in your rear working hard for you instead of your opponent. If your pushed back again, your new fort is just as defensible as it was in your enemy's hands.
If players were forced to repair it after capture, the victorious army has a choice of moving on, and leaving the fort vulnerable to recapture (Mongol Horde! Charge!), or staying and solidifying their hold on the province (Teutons!).
The problem with paying gold for repairs is that it means its cheaper to stay under seige if you have sufficient troops - unless, of course, your now paying to repair during a seige as well.
The reason i like the defense/repair mechanism is that its already in the game to some degree and so shouldn't be too difficult to implement.
As for the turns to build v repair, one could argue (perhaps not very well
) that its harder to repair than to build from scratch, and second that a slow free castle > fast costly castle.
[edit] Oh and it might help to not think to literally
. Walls don't just mean walls. Supply lines, observation Posts, magazines and auxilliaries, local contacts, peasant labor and supplies, all might be part of the generic 'fortress' we have here.
[ July 11, 2004, 04:28: Message edited by: SelfishGene ]