Most hotels in Spain ask for a credit card to guarantee the room anymore, but if you say you don´t have one, most are willing to give you the room on a 2pm or even 6pm hold, that is, they´ll tell you to call them if you are not going to arrive by that hold time, and once the deadline has passed, they´ll sell the room to whomever asks for it. I suppose this is more so in Hostales, since they get lots of people every day that show up off the street, without a reservation.
If that doesn´t work, just give them your debit card number. Most hostal employees won´t know the difference and they won´t try it unless you are a no show.
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Olga
Seville