Description of Bug and how to replicate it: Can't build watchtowers or forts / limited movement. Don't know how to replicate, just happens on some regions.
Probable Cause: Disaster event got stuck or something so the game thinks its still active many turns later.
Suggested Fix: Wait or cause another disaster in the region that clears up correctly.

Working Fix (workaround): Edit the disasters file to force a disaster event to occur, run the saved game for a turn or two so the event happens, save, and revert the disaster file back to the original.

The file is: data\world\maps\base\descr_disasters.txt

Make a copy of the file first, then edit the original and make a copy of the first entry, for earthquake, and delete the "climate" line. Replace that line with "Position" specifying the affected region. You need to lookup the position of the city with the affected region using "show_cursorstat" in the game console with the mouse cursor over the city.

Change min/max scale to 1 to reduce the effects of the disaster, and the year/frequency from 20 to either 1 or 2. I'm playing with 2 turns/year (0.50 time scale), so if you are using the default 2 years per turn, I would try 2 instead of the 1 that I used. These numbers appear to be in years, so with a 0.50 timescale, or 2 turns per year, it takes 2 game turns for it to occur.

Save the file (its read-only so you have to undo that property). Load your game, and run a turn or two. Hopefully an earthquake will properly hit and undo the watchtower/fort bug. Save post-earthquake and then go back and restore the original descr_disasters file.

--------
Lastly, I noticed that this oddly didn't work in the odd game turns using 0.50 timescale. That is if the saved game was during a particular season or 1/2 of a year, the disaster event will never trigger for some reason. Simply go back to a saved game 1 turn earlier seems to fix it. Guessing it only counts properly either every even or odd turn number... or if the game starts at a half year, it never triggers (i.e. year 1110.5).