There is a variant here which directs you to leave the N-634 before arriving into Mioño. It takes you to Castro Urdiales via the coast.
In Mioño the camino leaves the N-634 by turning left just before the church, following the signs to the Asador El Tunel. Just past the Asador (a bar and restaurant) is the Tunel, an 800m path cut through the mountainside. At the other end are the southern suburbs of Castro Urdiales, which you will enter along the N-634 (which went around the mountain as you went through it).