There will be guided visits to Ciudadela in Pamplona, every saturday till August 31st. Ciudadela is the oldest fortress in use at the moment. Actually, it is considered a gem of the militar architecture of 16th century, with a lot of copies in Europe and abroad.
Groups of 40 people maximum, and it costs 3 euro per persone. To book it, via mail (firstname.lastname@example.org) or by phone (948211554). Also is posible at the Tourist Information Center.