These famous towers form part of the Puerta de Serranos (Serranos Gate) in Valencia. This was one of 12 city gates, part of the city's ancient Christian walls. Nowadays, it is one of the city's most visited monuments, an imposing size; climb to the terraces at the top for amazing views. The most popular story about the tower is that it served as a prison for knights and nobles from 1586 until 1887.