The remains of the fortress can be visited
Oderberg Fortress

Oderberg Fortress, once known as Bardyn Castrum and popularly called Bärenkasten (Bear Box), was a border fortification. Originally located on an island in the Oder River, it protected a river crossing and trade route, while also ensuring customs control. Today, only remnants can be found within an allotment garden in Oderberg.
History+
Oderberg Fortress was constructed starting in 1353, after an earlier castle was destroyed in 1349 during a war against the Pomeranians. Margrave Ludwig the Roman of Brandenburg had the old structure demolished because its location no longer met the requirements of a modern border fortress. Instead, a new fortification was built on an opposing island in the Oder. As the Margrave lacked the financial means, he entrusted the construction to local nobles Henning von Uchtenhagen and Dietrich von Mörner, who in return received a share of the fortress's revenues and residential rights. After its completion in 1355, Henning von Uchtenhagen sold his shares. In the following years, the fortress was equipped with weapons, including cannons. Under its protection, Oderberg experienced an economic boom, reflected in the highest revenues in the Margraviate of Brandenburg in 1375.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: Roland Neubert, Halle (Saale)
This description draws on the facts documented in the linked Wikipedia article; Wikipedia content is licensed under CC BY-SA 4.0. Wording and editing: Mobile History.
Location: 52.8622, 14.0394