Castillo de Los Tres Reyes del Morro
Castillo de Los Tres Reyes del Morro, often shortened to Morro Castle, sits at the entrance to Havana Bay and is one of the most eye-catching sites of any walk along the Malecón. It was originally designed to protect the Spanish outpost in the 16th century. Various problems with the construction and politics kept the project from being completed till the 17th century. The main action the fort saw was during the 7 Years War, when the British bombarded and captured the fort.
Today, the walls of Morro Castle are well maintained, though the canons around the outer edges are largely rusted. There is an exhibit of the lighthouses of Cuba inside, and you can see a ceremonial canon fired at sunset from here on most days.