Quote Originally Posted by nDervish View Post
The upkeep would have to be a temporary thing, I think. Start it off at a very high level (comparable to the initial casting cost, even?), but taper it down to zero over the course of 20 or 25 turns. Otherwise, if it's a permanent drain on your mana just to continue holding on to the city, then the more sensible use is to Enrapture the city, move the defenders away from it, march in your own army, then drop the spell and let your army conquer/pacify the city non-magically. While that's still effective, it seems to defeat the point and flavor of a "capture town" spell - it's no longer something unique, just yet another way to nuke a city's defenders before the army moves in and does the real work.
This is along the lines of what I said, only explained in different words (and the more explanation the better).