Prehistoric ExtremaduraExtremadura possesses a real treasure trove of prehistoric monuments located in many different parts of the region.
Dolmens, which are megalithic tombs made up of a capstone that's held up by two or more upright stones, can be found north of Mérida, in the outskirts of Badajoz and in the area around the village of Barcarrota. However, there are 41 (sic) of them in a small area around the towns of Valencia de Alcántara and San Vicente de Alcántara, situated in western Extremadura by the Portuguese border.
The local tourist office has created trails that visitors can follow to track them down, and their leaflet on the subject is definitely worth getting if you want to find them. Many are open access, although they may seem at first to be hidden by vegetation, trees, etc. You can often even walk among the stones, which is a strange experience. The inevitable drawback is that vandalism is unfortunately always a risk.
This same thorny issue of access is true for many of the cave paintings that are located in mountainous areas throughout Extremadura (I've visited some in the hills near Hornachos and Calamonte, but there are several other sites too, such as Los Barruecos, a gorgeous drop of countryside that's discussed in more depth elsewhere on this website.
You'll have the buzz of getting up really close if you actually manage to find cave paintings in Extremadura, but once there graffiti can sometimes be seen scarring nearby rocks.