The Guaita fortress is the oldest of the three towers constructed on Monte Titano, and the most famous. It was built in the 11th century and served briefly as a prison. It is one of the three towers depicted on both the national flag and coat of arms. San Marino also has a cake known as La Torta Di Tre Monti (Cake of the Three Mountains/Towers), as a symbol for the area.
Source and more informations: The Guaita Tower