The Bear Tower - Jajce
The Bear Tower Jajce (Bosnia and Herzegovina) was built in Romanesque style, and being so indicated that it is the one of earliest military structures from the Bosnian MIddle Ages.

Thory of names is simple - due to its thickness of the walls which are reaching between 4.5 and 6 meters it was given such a name.

Also there is mythical one which says that it was named so because it was place where bears fed on captives and convicts, who were kept there. Pretty amazing.