Medieval tower of San Martin de Hoyos, in the municipality of Valdeolea (Cantabria, Spain) was built in the late Middle Ages (XIV century).

It had a dual mission first as watchtower against possible invasions and later as a custom control for paying fees for the entry of any product that would be sold in the region.

Its character of fortress is given by the moat that surrounds it and that can be seen even today.

This fortress-tower retains its elevation  although it is unroofed and emptied his interior.

Source

http://es.wikipedia.org/wiki/Torre_medieval_de_San_Martín_de_Hoyos