Castle of Avanta, also known as Potamos Castle, stands on a hill north of Alexandroupolis, above the road toward Avanta. The surviving medieval towers and double walls are thought to date from the 13th century, though the site has traces from earlier periods.
The climb takes about 20 minutes and is part of the appeal. From the walls you get a broad view over the plain, which explains why the hill was chosen as a fortified lookout.