On march the 14th at 18:00h (Spanish hour) was opened the reservation system for attendants of Euskal Encounter (the mega party that hosts RetroEuskal). I's not necessary to book unless you want a place for your PC, as you can read following.

It’s known that party seats become occupied quite rapidly, so if you want to book it’s important to quickly make your pre-reservation (basically to register yourself in the reservation system if this is your first attendance to Euskal Encounter) and then make the final booking.
For registering you can use this page. For final booking use this one.

Reservation is necessary only if you want a site for your PC (electrical power and ethernet plug for internal network and Internet access) during the whole four days party. Remember that every year there are also available one-day ticket for 10€, so you don’t have to pay the full 40€ fee if you just want to enjoy one or two days RetroEuskal or the rest of Euskal Encounter (this point to be confirmed by organization).