Aguilar de Campoo is the biggest town between Arija and Cervera de Pisuerga, with all services. It was walled in medieval times, and some vestiges remain along with houses bearing large coats of arms. The Rio Pisuerga runs along one side of the center with a pleasant promenade.
The Colegiata de San Miguel has Visigothic origins, though what stands now is mainly late Romanesque and Gothic — the choir stalls and baptismal font are highlights. But the real prize is the Romanesque church of Santa Cecilia, on the edge of town below the castle. Get the keys from the Casa del Cura across from the Colegiata — hand over your passport and you'll have the church to yourself. The capital of the Slaughter of the Innocents is exquisite, and the experience of unlocking a Romanesque church for a private visit is unforgettable.
Hotel Valentin includes breakfast. The Posada Santa Maria La Real, at the edge of town on the camino, is the splurge option — a converted monastery with a cloister. Aguilar also has a Romanesque institute with a museum, though its opening hours can be unpredictable.
The walking from Aguilar is lovely — lots of off-road and river walking. Head down the promenade to the monastery, continue to the reservoir, and go up and around on the right (don't cross the dam).
The first surprise is the anthropomorphic tombs just past Corvio, probably 10th century — the bodies were oriented with feet facing east. From there, lots of field walking, then at Salinas de Pisuerga you cross the Rio Pisuerga and walk alongside it for 12 km into Cervera. About 1-2 km before Cervera, a sign points to the Ermita Rupestre de San Vicente, about 500 m off-camino — a medieval rock hermitage with more anthropomorphic tombs. Highly recommended.