Just a wild guess, but could it be possible that your "view depth" got set wrong? (View Depth determines how far you can see in a level.)
Each of the four windows has it's own view depth setting. (When you restored from the CD, I believe you would have also restored your old view depth setting...)

Click on a window to activate it. Then press the plus key. Down at the bottom, it should tell you (in quants) how far you can see. Keep pressing the plus key - those numbers will get bigger, and you may start to see things re-appearing. Do that for each of the four windows. The plus key expands how far you can see; the minus key contracts the view.
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On the other hand, if you tried to move right up next to the fort, and couldn't see it, then this probably isn't the right answer....