Suur rannavarav (Great Coast Gate) was the way from the city to the harbour. The outer gate of the 14th century coastal gate was reconstructed from 1518-1529 and a squat artillery tower up to 25 m wide arose next to the slender sentry tower.
It only got its nickname, Fat Margaret, in the 18th century. The thickness of the lower part of the walls is up to 6.5 m; it gradually becomes thinner farther up. A small city seal is carved into the harbour side in late Gothic style (1539).
The tower was also used as a gaol. The maritime museum has been here since 1961. The four floors of exhibits survey the history of Estonian shipping: shipbuilding, harbours, voyages, navigation and lighthouses, fishing, oceanography and marine science. The rooftop affords a fine view of the bay and the Old Town.