As its name indicates, this Tower, built in the 10th century, and with a 14th century arch, has a round configuration, which makes it unique among the other towers of the castle. 

But the uniqueness of this tower goes far beyond its cylindrical shape. It is a tower with two profiles: a square bottom and a circular top.  It has a horseshoe-shaped plan, which meant that during the restoration work, the upper vault had to be dismantled and done again. Another unique feature is that it is the only tower where an underground gallery could be accessed from the inside.

In this tower was the underground cistern, which allowed it to have water, in fact it is the only tower with a cistern.  In addition to this it had two more floors, one larger and one smaller, both accessible by stairs. Access to the lower floor is through the courtyard of arms and access to the upper floor is via the adarve. Access to the top was via interior stairs from the second floor.