Grzybowo is one of the largest strongholds which came into existence in the period of the formation of the Piast state. It was constructed 6 km to the north-east of Września. The stronghold, well-visible in the field, takes up an area of 4.7 ha. Its shape is close to a rectangle with rounded corners. The stronghold is surrounded by a rampart, whose remains are up to 9 metres high. From the east there is a depression in the rampart, which suggests that the main entrance to the stronghold was once located here. The other entrance is outlined on the northern side, where two arms of the rampart converge, slightly overlapping each other. The beginnings of the stronghold in Giecz can be dated to the 920s and 930s. It seems that a small stronghold in the north-western part of the present premise came into existence first. This stronghold had a fairly regular shape, with an outline close to a circle. Its internal diameter was 40 m and the core of its rampart was about 17 m wide. A conversion of the stronghold started in the mid-10th c. It consisted in a levelling of part of the terrain outside the original stronghold and using a much larger area than before. It was then that the ramparts whose remains we can see today were constructed. The last stages of the conversion of the stronghold took place in the early 11th c., and the end of the permanent inhabitation of the stronghold can be dated to a period no later than the mid-11th c. On the other hand, some settlement in the stronghold still existed in the 12th c.

The most valuable finds displayed in the permanent exhibition include weaponry (mail hauberks, javelins, arrowheads), and artefacts of daily use, such as knives, needles or awls. There are also finds which were considered luxurious in the Middle Ages: antler combs, bone fittings, and fragments of jewellery from glass, amber, carnelian or rock crystal. The exhibition’s greatest treasure is a hoard of coins and silver ornaments.

Inside the museum building, which was constructed in 2010, a unique exhibition “The Stronghold in Grzybowo – between the Tribe and the State” was organised. It imitates an archaeological trench and explains interpretation methods which allow for a reconstruction of the discovered remains of the stronghold.

Every third weekend of August the stronghold welcomes the participants of the annual historical reenactment event called the Warrior Tournament of Grzybowo.

In the Museum there is a sales point with souvenirs and museum publications.