I have picked these hostels in these cities, so if you have stayed at any of these hostels or heard anything about them please let me know:

Madrid: Hostal Metropol
Seville: Hostal Redes or Hostal Nuevo Suizo
Granada: Hostal AB
Barcelona: Kabul Hostel or Gothic Point Hostel

If you know about the location of these hostels, please let me know. I want to be near the sights and nightlife. Thanks!!