There is not much left of the Hrodna's 'old castle', most of which was destroyed during the Great Northern War in the early 18th century. You can wander around the remains of the castle, with great views of the Neman River below, but the main attraction on the castle grounds is the Hrodna History and Archaeology Museum. It's a comprehensive walk through the history of the Hrodna region.
English placards accompany most exhibits. Highlights include a set of mammoth tusks, neolithic spears and arrowheads from 3 to 4 BC, an 18th-century dugout boat, and armour and other war paraphernalia from the Lithuanian period. There's a fine diorama of Hrodna in the 19th century and the requisite WWII displays of weaponry and Soviet propaganda posters.