Prior to setting out into these valleys, one should mention Torrelavega, where the the rivers Saja and Besaya meet. Its oldest building is an 18th-century casona in Los Mártires street. More interesting are the examples of 19th- and 20th-century architecture. As for religious architecture, Torrelavega has two very different instances, each of them very valuable: the church of Asunción, carried out with historicist rigour, and then the church of Virgen Grande designed by Luis Moya. There are notable examples of lay architecture such as the Town Hall, the palace of Demetrio Herrero in former times. In spite of the extremely rapid urban growth linked to industrial development, in the environs of the Plaza Mayor (main square) and the old plaza "del Grano" streets still remain that retain the charm of the old town, which by an edict of the Regent María Cristina in 1895 was granted the status of city.

The Besaya valley, the natural thoroughfare from the Meseta uplands down to the coast, still has remains of the old Roman road: along the upper stretch of the valley, between Bárcena and Pesquera. This road originally connected Herrera de Pisuerga in the province of Palencia with the Roman town of Portus Blendium (present-day Suances). As a natural thoroughfare between the coast and the interior, this route featured churches and monasteries of the Romanesque period of which a good number remains. One can travel a Romanesque route starting at the head of the valley and descending to the church of Santa María de Yermo near Torrelavega. These Romanesque churches are small, of one single nave, and have the Gothic groin-vaulting that replaced the original wooden roofing. The first of the churches is the church of Aldueso near Reinosa, the second the church of Rioseco, which has an interesting frontispiece; going down the Hoces de Bárcena route one finds, very close to the bank of the Besaya, the church of Bárcena de Pie de Concha, which has later additions to it. Nearby is the church of San Martín de Quevedo. The church of San Facundo and San Primitivo, in Silió , is of a higher distinction than the ones mentioned previously. The most interesting thing about this church is its apse. Finally, there are the churches of La Serna, San Juan de Raicedo and Santa María de Yermo, the most important of all, situated near Riocorvo; its remarkable feature is its having one of the few decorated tympanums in Cantabria, depicting St George slaying the dragon. In the nearby villages of Cartes and Riocorvo there are two linear neighbourhoods with important casonas to either side, some of them bearing coats of arms in stone.

In the Iguña valley, before coming down to Yermo, one should remark on the palace of Hornillos, which foreshadows 'picturesque' architecture in the English style in Cantabria and the construction of the Palace of La Magdalena in Santander.