Paks Margareeta (Fat Margaret Tower) and Suur Rannavärav (Great Coastal Gate)

From www2.tourism.tallinn.ee:

The Great Coastal Gate (Suur Rannavärav) and Fat Margaret's Tower (Paks Margareeta) were built as a defense on the seaward side of town, but also for impressing visitors coming in from the sea.

The Great Coastal Gate, built along with the city wall, is situated on the northern side of the Old Town, near the harbour.

During the reconstruction of the gate in the early 16th century, the cannon tower Fat Margaret was added. The round tower, with 155 loopholes, a diameter of 25 meters, and a height of about 20 meters, was built to protect the harbour. It got its name from the fact that it was indeed the stoutest tower in the city wall. Through history, the cannon tower has also served as a storehouse for gunpowder and weapons, and as a prison.

Fat Margaret's Tower now houses the Estonian Maritime Museum, with a permanent exhibit on Estonian maritime and fishing history. The viewing platform on the roof affords a lovely view onto the Old Town and the bay.