The First Tower (also called Rocca or Guaita) is the major and most ancient of the three towers overshadowing the City of San Marino.

The central donjon, former watch post and shelter of the early inhabitants of Mount Titano, dates back to the 11th century and is therefore one of the most ancient fortifications in Italy.

The First Tower is limited by two circles of defensive walls. The inner ones, adorned with merlons and braced on the corners with angular towers, subsequently lowered during the 16th century, belonged to the first wall circle (Guaita Wall Circle), which was built to defend the inhabited area. The inner walls, the most ancient ones, are characterised by a raised entrance and include the Bell Tower, the cells of the garrisons, subsequently used as prisons, and the donjon, the ancient watch post which was rebuilt during the second half of the 15th century.

Traces of subsequent restructurings are still visible: for example, a dagger sculptured on one of the ashlars near the bell tower and an inscription in gothic characters on the eastern tower have been recognised as symbols left by the Comacine Masters, who supervised the first restructuring in the 13th century, while the dates 1481 and 1475 engraved on the arch of the entrance door and on the lintel of a loophole respectively refer to the important restructuring works carried out in the last decades of the 15th century.

However, the numerous works, mainly in the 15th and 16th centuries, did not substantially alter the aspect of this defensive complex, which still remains austere and unadorned.

Thanks to the last restructuring around 1930, the First Tower was opened to the public.