Byzantine writers call the Proto-Bulgarian fortresses αὐλαὶ - palaces and courtyards. From there in the Bulgarian scientific literature, the denomination “aul” was established. Archaeological excavations in northeastern Bulgaria can give a clear idea of these fortifications, as centers of administrative and military power of the Bulgarian rulers from the early Middle Ages. The auls are fortified palaces. The army was stationned in them and a small palace building was erected. In some of the larger ones, the temples of the supreme Bulgarian god were erected.
The purpose of the fortresses located in the interior of the Bulgarian state was to strengthen the defense of the capital Pliska and to be used as a shield. This broad fortification and the deployment of the permanent khan's army among showed its purpose in the fateful summer of 811. In his campaign against central Bulgaria, the Byzantine emperor Nikephoros Genikos encountered these Bulgarian fortresses, and although he captured and burned many of them, he could not destroy the striking force of the Bulgarian army. Due to the resistance of the garrisons of the villages, the Byzantine army moved relatively slowly in the Bulgarian lands. Thanks to this, Khan Krum managed to withdraw then regroup his main forces, before facing the Eastern Roman Empire and defeating them in the Varbitsa Pass.
One the most famous auls remains the one built by Khan Omurtag on the river Tisza in the first half of the IX century. Another similar one was Preslav. Before becoming the royal palace of the Bulgarian state in the time of Simeon, it was part of the defensive line of Pliska. The very capital of the first Bulgarian state in the early beginning was an aul, the original seat of the central Bulgarian army. Subsequently, after the permanent establishment of the ancient Bulgarians, it expanded, with a number of administrative buildings, markets, neighborhoods that appeared and gave it the appearance of a city.
One such early aul designed to guard the road between Pliska and Drastar was discovered and some of its sections were excavated and studied, near the village of Kladentsi, Tolbuhin district. No traces of a palace were found there in the true sense of the word. The first dwellings of its inhabitants were yurt-like buildings. It was only later that they began to make dugouts to replace them. This village is a quadrangle with sides of 160-200 m. It is fortified with a defensive mound and a moat. It was built on a site where there were numerous wells with abundant water in this once waterless area. This gave the fortification a special significance. Most important was its key location right in the middle of the road between the central “ongal”(fortified camp) of Pliska and the important Danubian fortress of Drastar (now Silistra). The army stationed there was tasked with defending the numerous open settlements in the river valleys facing the Danube between the forests and the steppe. Drastar itself was included in the country's defense system. This is evidenced by the Bulgarian apocryphal chronicle from the XI century, where it is written: "And this king Ispor (Khan Asparukh) created great cities: Drastar city on the Danube" .
The parts of the fortress walls of Drastar discovered during the archeological excavations show that the Bulgarians of Khan Asparukh have rebuilt the parts of the old fortress wall still preserved and have completed them. The new buildings had a very impressive and monumental character, which is why in the memory of generations until the XI century it is preserved that Khan Asparukh created the city of Drastar.
The auls of the Bulgarian rulers were an expression of strength and authority, both for their own people and for foreigners. Apart from defending the country from enemies during wars and trials, they carried with them all the achievements of Bulgarian culture and spirit. The houses and buildings in the villages were decorated with wood carvings, ornaments and symbols closely related to the life and spiritual culture of the ancient Bulgarians. Through them, the population living in the vast territory ruled by the Bulgarian rulers gained the awareness that they were part of a whole. Thanks to them and the spiritual centres or various administrative units they hosted, the triumphal victory columns that the Bulgarian statesmen erected in them, the process of consolidation and transformation into a whole of the ancient Bulgarians and the local population went smoothly. In this way, the auls helped to form the national self-consciousness of the Bulgarian people.