One or more of the world.* files in your save folder has become corrupt.
The world files keep track of the walls and ground, quest opened locations and items left directly on the ground.
If you delete the effected world files, the game will replace them with the default versions the next time the save is loaded. That should fix the walls, but may cause other problems (though they have workarounds, see below). If you have any recent saves that work properly (doesn't sound like it), it you could copy the world files from that save into your most recent save.
Different file extensions are associated with different areas of the game;
.0 / .x0 = Aleroth, Ars Magicana, etc
.1 / .x1 = Verdistis, Council Hall
.2 / .x2 = Dark Forest
Where are you in the game (ie where have you explored and what quests have you done)? I have all my old saves, so can get the relevant world files from as similar a point as possible, to minimize the problems you may have.
Since resetting the world file will remove items left directly on the ground, you may need to use the inventory file procedure to replace any missing quest items;
Lost Items - guide and directory of inventory filesFor any places that get resealed, or doors that get relocked, you can use the following to get back in;
FIX for the Missing Catacombs PortalTeleporting Anywhere, via hex editorAlso, there can be a small gap at the north west corner when 2 walls meet, that you may be able to toss a teleporter stone through. This can also be done with some gates and house windows.