How about ... a bit out of the way, but ...

Santa Cruz - Cangas de Onis, 60km E of Oviedo : a decorated chamber covered by a large tumulus upon which is built a chapel. The monument is preserved below the chapel and can be reached by a ladder! The decoration is painted and carved.

http://www.el-caminoreal.com/monumentos/dolmensntacruz.htm#

http://www1.las.es/~fmiranda/la%20historia.htm