A grad is an enclosed place protected against intruders. Most Polish grads were built on hills, in riverside swamps or on islands. "Ostrów" is an old Polish word for "island". The Poznań grad was named Ostrów Tumski (Cathedral Island) and was actually built on an island created by Warta and Cybina river backwaters in the 9th century. The more important, residential part was on the west side. This is where the seat of the Duke, together with the chapel and the main farm buildings directly serving his court, was located inside a strongly walled palatine hill. This area was surrounded by an earthen and wooden embankment. A more spacious, circularly fortified settlement adjoined the grad to the east. The cathedral, which was Romanesque even then, was the most elegant building. Ostrów Tumski takes its name from its cathedral (“tum” in Polish).
The binding rules of the day demanded that a stone bench foundation be laid down first. “Boxes” made of tree trunks were placed on this and filled with stones and sand. Elaborate construction in those days was reinforced with suitably arranged tree stumps, often joined together with "hooks" created by parts of branches. A high embankment was then created by covering all this with earth and clay and placing a wooden palisade on top.
The Poznań grad expanded over time and eventually became a little cramped. The next adjoining - and very spacious - settlement was created south of a muddy gully (in present day Zagórze) and partially surrounded by a protective embankment. This was where artisans and merchants resided. Presumably, there was also a garrison housing soldiers in the king"s service.
Things remained like this until the middle of the 13th century when Prince Przemysł I decided to establish a new city on the left bank of the Warta and to include the existing settlements in it. The defensive embankments on Ostrów Tumski disappeared but the Gothic Psałteria (Psalter House) was built in the 16th century. The island remained enclosed by a defensive wall (the foundations of one of its bastions was only recently discovered), a new Gothic cathedral replaced the castle chapel and the Church of the Blessed Virgin Mary was erected in the 15th century.
The Gniezno grad was just as large as that of Poznań. Giecz (Środa Wielkopolska County) was another strong centre of Piast power and the early medieval reinforcements of its huge embankment are still preserved. The strong ramparts of the grad on Ostrów Lednicki (Gniezno County) had a special significance. The grounds of the grodzisko (a Slavic hill fort) in Grzybowo (Września County) have also been examined. Grodziskos, or the remains of former grads, can be found all over Wielkopolska today.
The major Wielkopolska grads also served as temporary seats of the sovereign. They had to contain walled “palatiums”, or palatine hills (whence the word "palace"). While these early medieval "palaces" were a far cry from the beautiful residences of the renaissance or the baroque, their significance to the state was enormous. A palatine hill bore testimony to the strength and importance of the sovereign to whom its costly construction was dedicated.
The princely palatine hill on Ostrów Lednicki was one of the most important in Wielkopolska. Built on a rectangular plan during the second half of the 10th century, it was made of stone and presumably had several storeys with presentable and habitable rooms. A round rotunda and chapel adjoined the palace on a longitudinal axis.