Simply amazing place. We walked in on the off-chance, we got to Caso Viejo quite late on a Friday night, with other places already beginning to close. What a pleasant surprise, this place was the best one we ate it in both Bilbao and San Sebastian over four nights (not counting sit-down restaurants). The staff were unbelievable. I guess the main factor here like it is across Bilbao is just how fresh and well preserved the ingredients: the cheese tastes of cheese, the squid tastes of squid. I also loved the staff. I will always remember how they accommodated me and wife by whipping up squid ink croquettes since neither of us eats pork. It was touching, as was the time the proprietor/chef (??) came around to speak to have a chat with an old customer she knew well. She really took a lot of pride in the place and it was awesome.

There are obviously many places to eat in Casco Viejo serving broadly similar food, but if you don't know which to pick you can't go wrong at this place.