At the beginning of the siege, Mehmed sent out some of his best troops to reduce the remaining Byzantine strongholds outside the city of Constantinople. The fortress of Therapia on the Bosphorus and a smaller castle at the village of Studius near the Sea of Marmara were taken within a few days. The Princes' Islands in the Sea of Marmara were taken by Admiral Baltoghlu's fleet.^[10] Mehmed's massive cannons fired on the walls for weeks but due to their imprecision and extremely slow rate of fire, the Byzantines were able to repair most of the damage after each shot, mitigating the effect of the Ottoman artillery.^[11]