With Celtiberian roots, Tiermes dates to the 1st century AD and is one of the most important Roman sites in Castilla y León. The pretty 12th-century Romanesque Iglesia de Nuestra Señora de Tiermes that watches over the site serves as testament to its use down through the millennia. Traces of the Roman town remain, including what's left of the houses, mosaic floors, forum and aqueduct, all make this a worthwhile excursion. It's 7km southeast of Montejo de Tiermes.
There's a useful English-language guide to the site on the website.