Fortresses are an extremely important and cheap upgrade for your provinces. They provide a morale bonus to your people and a defensive bonus to your troops which rise with the level of the fortress. Furthermore, if the fortress is level 2 or greater, the strength, morale, numbers and composition of troops in the city will be hidden from view when stationed in the Fortress.
Ranged units are often preferred to take down the forts with and using infantry bulked with tanks will ensure that your infantry will take less damage (as tanks soak up a major part of the damage). In late stages of the game a large quantity of artillery (10+) or a large army with a few tanks will obliterate even level 5 fortresses quite quickly, but being able to hide your troops from enemy vision is nevertheless an important advantage even in late game.
The defensive bonus of a fortress should not be underestimated; Level One fortresses reduce damage dealt from attacking armies by 67%, while Level Five fortresses reduce it by 91%. In general, the damage reduction obeys the following formula (However this only applies if the fortress is undamaged and the fortress may well get damaged during battle. The damage done to the fort diminishes it's effect) :