I check Gronze when I'm not taking a group, and I use booking.com as well.
I know some folks don't like booking.com but so far, no problems and I've been using them for many years.
Though I LOVE Monastica Pax in Leon. Hostal Alvarez. Principia Suites. Aparthotel FC Catedral. Inn Boutique Leon.
In Pamplona, I like Hostal Bearan. In Old Town and not too expensive. Bar/restaurant below. There is a good cheap twin room at Albergue de Pamplona-Iruñako. Aloha Hostal is good. If you don't 'mind staying a bit out of Old Town (maybe 1.5 k) Alojamientos Olga is quite nice and a short walk back to Old Town. There are restaurants close by.
In Burgos, Hotel Norte y Londres is good. Hostal Cuentame Evolucion is good. Pension Boutique Doña Urraca is good. Boutique Museu Burgos.
In Leon, I also stay at Monastica Pax but there ARE other choices.
Have you thought about renting an apartment? Sometimes they're less expensive and you have a washing machine, kitchen, and in the end, they're a good value.
Pay attention to how far they are from Old Town when you book.
Also be sure to book lodgings with free cancellation.