Found it by looking at the hostal options once there.
Clean bathrooms (my bench mark)
For the price there is no better other than the hotel Don Alfredo.
All others are overpriced. Other than that I would suggest the hotel Melia which is nice and I started there.

When we arrived, Hostal El Castillo seemed like most of the other hotels/hostals we'd stayed in over the past couple of weeks. Clean, quaint, cute, affordable. It would get a great rating if not for two major issues we had while staying there:

1) The NOISE! Our room faced out onto the plaza that the hotel is in, and somehow the acoustics of this plaza make it sound like people are using megaphones. Quite literally - the first day we were there I actually looked out the window to see if the woman next door really was using one. And for some reason, people love to do very noisy things in this plaza, such as rev their motorcycles or ride them around in ciricles, and vacuum their fricking awnings. And if you're there during the week, you'll be awakened by 8AM by the children gathering for school, which is right next door.

Whether you're trying to sleep in past 8, get to bed before 1AM, or take an afternoon nap, there is ALWAYS something to distract you. And since there is no air conditioner, you really can't close it out unless you like a hot, stuffy room. By the way, my apartment in the US is right on an interstate, and my bedroom window opens right out onto the traffic - so I'm used to sleeping through noise. This place puts the interstate to shame. Maybe if you get a room that doesn't face the plaza, it's less of an issue.

2) Speaking of putting to shame... The hostal didn't provide a bathmat as one of the towels. So we used one of the hand towels as a bathmat, since we didn't want to track water all over the room when we got out of the shower. So at some point we must have stepped on it with our shoes on, and got a splotch of mud on it. The next day, we noticed that although housekeeping has been through, they took the old towels and didn't leave new ones. So I headed downstairs and asked the woman there if we could have new towels.

She went into the office and brought out new towels, but also brought our old ones. She proceeded to pull out the towel with the splotch of mud and lecture me about it! My Spanish is not quite good enough to get everything, but the gist was that the spot would be impossible to get out and that we needed to be more careful with their towels. She seemed quite angry and made me feel guilty to have soiled their towels... but at the same time, angry because there is no reason a hotel should make its guests feel guillty about getting some dirt on the towels! These aren't nice fluffy fancy-hotel towels - they're Motel 6 level towels and if they're paying more than 2 euros each for them they're getting ripped off! There's really no reason to treat your guests like small children, forcing them to come ask for new towels just so that you can tut-tut to them over the dirty ones.

So if you're looking for a noisy environment and a good lecture, this is your place. Otherwise, I'm sure there are other hostals in the same price range that won't condescend to you and where you can actually get a good night's sleep. I'd like to emphasize that I am NOT picky about hotels in general - I stay at Motel 6 regularly, and before this place I'd just spent four nights at the Velleda in Rabat, Morocco - a little place where they only have hot water for an hour a day. This is the first time I've ever had an experience at a hotel that prompted me to write a truly bad review for it.

Clean, cheap (EUR25 per night) and central. Staff friendly and helpful. In lovely old building with wooden beams and shutters on the windows that open out to a view of the mountains. I'm staying there again in 2 weeks.

Hostal Castillo is cheap, but charming and clean, still very Spanish, which is rare in Marbella. The fact that the staff doesn't speak a word of English probably has something to do with that. Especially good for young couples looking for a romantic place that doesn't cost them too much. There are also disadvantages (there's a school at the square opposite the hotel), but then again, each and every place in Spain will be noisy at times, won't it? Bas.