Sieges in real life can take months or years before the offense wins or gives up, especially in the middle ages before the advent of cannons. In modern times, it requires numerous air strikes as well as a ground offensive to take a well fortified city, and even then it takes awhile. Look at the mess in Libya. Destroyed fortifications are constantly being rebuilt and supplies are smuggled in. If you could take a well fortified city in a turn or two, it not only wouldn't be realistic, it'd be pretty boring as well.