Lubart's Castle is Lutsk's headline sight and one of the most complete medieval castles in Ukraine. The brick fortress took shape in the 14th century under Liubartas, the Lithuanian prince who ruled Volyn, and its three main towers still hold the old town together visually.
The appeal is not only the walls. The castle compound holds small museums, archaeological remains, old bells, weapons, and views over the city from the towers. It is the place to start in Lutsk, both because it is the strongest attraction and because the surrounding streets make more sense once you have seen the city from above.