Fortresses are large, heavily fortified structures that are used to protect a city or area from attack. Two common types of fortresses are citadels and bastilles. A citadel is a fortress that is located within a city and is usually the last line of defense against an enemy. A bastille is a large, fortified structure that is built outside of a city and is used to protect the city from attack. Both citadels and bastilles are designed to withstand long sieges and are often equipped with powerful weapons.