Hi
@Lihaseso! Do you mean when you are walking the Camino and passing through Bilbao? If so, there is an excellent metro service that many pilgrims take to avoid city walking.
There are many stations throughout the city - the station in Casco Viejo (also named Zazpikaleak) is right underneath the stairs that you walk down on the Camino route! From there, you can take one of 2 metro lines (depending on which side of the river you want to be on).
One will take you to Portugalete (station name), where there are Albergues. The other line is on the other side of the river. If you get off at the station called Areeta, it is a short walk to the hanging bridge - which many walkers like to take - to Portugalete.
Buen Camino!