What is this tower I found in Spain?

//ads -- adsterra.com -- native banner
?>
42º37'46.6"N, 2º5'12.15"W
No openings on three sides. Door and windows face south. No other openings except ditch from underneath, going north to point straight at Azqueta, but it only goes a short distance. Seems at one time to have had a pipe going from bottom to top. No signage.
Pipe makes me think of water, but it is on top of a hill, and it is a long way from any point higher or to any village.
Pictures about "What is this tower I found in Spain?"



Why 70% of Spain is Empty
Sources: Stack Exchange - This article follows the attribution requirements of Stack Exchange and is licensed under CC BY-SA 3.0.
Images: Mehmet Turgut Kirkgoz, Mehmet Turgut Kirkgoz, Olena Ukraine, Mati Angulo