How to get here
The best arrival airport is Bilbao. Castro Urdiales is 45 km from Bilbao airport, and can be reached from there by either bus or taxi.
From Bilbao to Castro by bus: Regular buses from Bilbao to Castro leave from Bilbao Termibus (Bilbao main coach station). To reach the termibus from the airport, see below. The company Bizkaibus-Encartaciones (tel:946 363 424) connects Bilbao to Castro. There is a bus every 30 minutes and the trip takes some 50 minutes. Price is about 2.50 euros. This bus makes several stops in Castro. You should ask the driver for the best stop to the Hotel Las Rocas and/or Miramar (or to the "Playa Brazomar", the nearest beach; see the street guide).
From Bilbao airport to Bilbao Coach station: To go from the airport to the Termibus, take either a taxi (~20 euros) or a bus that runs every 30 minutes (price: 1.50 euros, ride: half an hour) more details here.
From Bilbao or Bilbao airport to Castro by taxi: around 50 euros. You normally do not need to reserve in advance, but can do so calling 944800909
The company ALSA (tel: 902 422 242) has direct buses to Castro Urdiales (coach station, about 2 km from the CIEM and Hotels) from many cities in northern Spain including Oviedo, Santander, San Sebastián, Vitoria, Pamplona, Zaragoza and the french border (Irun-Hendaye). Although probably less convenient, there are also direct buses to Castro from Barcelona and Madrid.
You can reach Bilbao from Madrid (5 hours), Zaragoza (4 hours), Barcelona (7 hours). The national railway company is RENFE.