If that doesn't help, check the '..\SteamApps\common\Divinity - Original Sin\Data\Mods' and '..\Documents\Larian Studios\Divinity Original Sin\Mods' folders, and delete/move any contents.
That error can also be caused by a missing configuration file (though that wouldn't account for the missing walls, etc).
Try starting the game directly from the '..\SteamApps\common\Divinity - Original Sin\Shipping\EoCApp.exe' program file, by right clicking and running as administrator, then change a sound or video setting and exit.