The Great Moravian settlements can be divided into four main categories. The most important were localities with central functions like Mikulčice, Staré Město - Uherské Hradiště and Nitra, where several castles and settlements formed a huge fortified (pre-)urban agglomeration. Along with the main centres, the system of fortified settlements included fortified regional administrative hubs, forts whose primary function was defence, and refuge forts which were not inhabited permanently but were used in the case of danger. The largest forts were usually protected by a chain of smaller forts. Smaller forts were also built to protect trade routes and to provide shelter for peasants in case of attack. The existence of noble courts like in Ducové and in other places is also documented. Their form was probably inspired by Carolingian estates called "curtis".