Question is in the title
Yes, only rockets level 4 and nukes ignores the forts.
Browser games are an ingenious business idea to lure out money -
- more or less cleverly camouflaged as a real game sometimes.
So beware of caltrops, spring-guns and booby traps.
Warning! Texts above this signature may contain traces of irony!
They also work as a great multiplier, lets say you are being attacked a couple weeks into the game. You could use a strategy of anti air, commandos and infantry in a city that has level 5 fortifications. You have a great Defense bonus against the bombers, it would greatly reduce a artillery attacks effectiveness and having the city bonus as well you could defend against tanks. Throw in some artillery yourself to punish them as they move in and then you can reduce their army strength and then counter attack.
They also do well in mountain provinces when combined with militia if you are short on steel while in a fight.
Just don't build them everywhere because 3k steel each could be better spent on attack forces most of the time.