Something similar was tried in a experimental game called one-fort. Where no one was allowed to build more than their starting fort; the effect was the same, the whole game went by with noone conquering another player and just turtling up the little area around their fort, very boring...then people quit.
I myself am just finishing up a game where I messed up early and ended up in a perpetual-besieged-state until late-game, controlling only my fort and maybe an adjacent province from time to time; that sucked too
