This little fortress was built in the 18th century in order to keep watch over the coast and defend it from pirates such as the well-known Cabeza de Perro (Dog's Head), who was born in Tenerife. This area was often attacked by the pirates, not least because of the massive trade of Malvasia wines. The fortress is well preserved and you can still see three of the five original canons. 

The spot was used to shoot several international films such as the 1st version of Moby Dick and James Bond.

Source:
https://www.webtenerife.co.uk/tenerife/the-island/municipalities/realejos/places-interest/fortin-san-fernando.htm