That's the fortify bonus, in civ 5 its currently pu to 50%, fortify for 2 turns, get 50% one turn = 25%.
And a unit fortified in a fort adds another 50% meaning a total 100% bonus
And sayy... if you have a river in front of your fort, go ahead nd tack on another 50% if enemy attack the fort across from the river.
And finally, if it's in a forested hill.. that' sjust evil
My forts never get attacked by the npcs, for a good reason. They'll lose the battle if they did that. xD
I don't understand the hate on forts. They assert positional control in a region. In fact, current game have 3 forts atm. One to keep spainards skittish about attacking rashly, One that was intended for Indian forces but ended up being used against french cuz india died to france.
And a third inactive but defended fort against barbarians and possibly korean troops.
In fact, Spainards was gonna attack me for the third time, but after they saw the fort built, they was like, eh screw this crap lets go somewhere else!