Players typically treat forts as infinitely undesirable within 6 tiles. For example, if a fort is just far enough from some houses but will affect a road beside them, then the road is plazaed before the fort is placed. But when I'm in that situation and money is tight, I'd like to delay plazaing if possible, so the exact desirability effect is important.

A fort does not have the desirability effect (-20, -20, -18, -18, -16, -16) that would be expected from Pharaoh_Model_*.txt files. Instead, both the 3x3 "building" and the 4x4 "parade ground" appear to have that effect, so that tiles within range of both parts get a net desirability of -32 to -40.