Wolves does almost the exact same thing….. However I do see the point.
“Ships can only eliminate this forts flags on a cannon roll of 6. Musketeers or Marines can eliminate up to two flags on this fort when they hit.”
Would the above work better as an ability, if nothing else is changed about the fort?
Wolves works because it affects all players equally, usually shows up semi-randomly on an island, and the effect can be ended with a single shot.
But, yes, that adjustment works. It’ll still be a very tough fort to beat, but not invincible without those specific abilities.