The above is one of three spike strips with big nails in its upper part and two huge metal rods below as an anchor.
And be warned that Galicia is one of the areas of Spain where the presence of traps in the forest has been reported often in the past couple years, including the above impaling spear
https://www.todomountainbike.net/art/alerta-en-galicia-trampas-para-ciclistas-en-gondomar-pontevedra