Before launching his final stroke against Thessalonica, Theodore also took care to secure his southern flank, by appointing his brother Constantine Komnenos Doukas as governor in Aetolia and Acarnania. An energetic governor, Constantine not only effectively shielded the Epirote domains from the threat of the Duchy of Athens, but soon recovered Neopatras and Lamia as well. Theodore himself turned his attention to clearing Thessaly of any remaining Latin presence, culminating in the surrender of the great Platamon Castle in 1218. Over the next few years, one by one, Theodore captured the fortresses around Thessalonica itself. Platamon controlled the entrance to the Thermaic Gulf, and with the surrender of Serres in late 1221, Theodore cut the land connection between Constantinople and Thessalonica as well. Thessalonica was left thereby, in the words of John Fine, "more or less an island in the midst of Theodore's possessions".