A superbly located flat on one of the 'Seven Streets' in the Old Town. It is hard to over-state HOW BRILLIANTLY CENTRAL IT IS - everything is in such close proximity, it's a proper base from which to explore (and return to). The apartment is very well furnished, with a beautiful bedroom and balcony overlooking the street. The living room is also comfortable if, for some absurd reason, you are spending any time there, but generally this WILL PROBABLY only be at the end of the day when you are exhausted. There is also a well-appointed kitchen (with coffee machine) but again, you should be out at a pinxtos bar.
A word on the noise, which some reviews have commented about below. We stayed Thursday through to Sunday morning. There's no getting away from the fact you are staying on a popular, bustling street. The crowd was nothing that wasn't manageable heading towards the weekend, it was great feeling part of the city, and the windows insulate out most of the noise. Saturday was much busier but nothing that exhaustion from all the walking (& eating) and ear plugs wouldn't fix. Don't let that deter you from staying in a flat in the perfect jumping off point for Bilbao.