In the village of Retortillo, on the road going south of the Ebro reservoir close to Reinosa, are the Roman ruins of Juliobriga, thought to be one of the most important Roman settlements in Cantabria. 

 The site consists of several parts.  Firstly, on arrival in the village you drive past some of the ruins, both to your left and right, before the church.  Just past the church is the vistor's centre, with a large car park.  The visitor's centre runs in conjunction with two others, in Villacantid (where there is a museum about romanesque architecture) and Camesa (where there are the remains of a Roman villa and interesting displays on the history of Romans the area).  There is a join entry fee for all three of around 3 euros.  At Juliobriga, however, the ruins are not cordoned off, and it is perfectly possible to go and walk around them for free.  There are clear information boards about each of the ruins, of which there are two main areas.  Firstly the area on either side of the road around the church, and also walking on past the church down the track are what are maybe the most impressive ruins of a villa complex, with columns and obvious water features.  From here there are also wonderful views back over Reinosa and to the mountains beyond.  More views can be seen by climbing up the church tower (up the steps), up to the bells - don't touch them though, you'll wake the whole village!