Generally I’d stay clear of large, empty developments with lots of properties for sale at heavily reduced prices. Unless you intend to actually live in the property you are unlikely to sell it, rent it, or do anything else with it apart from pay fees and taxes on it.
If you want to take “pot shots” with bargain properties then I guess you need to look for areas where demand might suddenly increase for some reason. The problem with Spain is that as soon as there is an upturn in demand anywhere, the developers rush in and create an oversupply. If you can find places where they can’t do that because it’s in a city undergoing regeneration (e.g. because an AVE station has just been built there) or because it is highly protected from over development (Menorca comes to mind) then there might be opportunities (I’m not holding my breath though)