The food and drink in the Plaza De Castillo are extremely expensive, and not exceptionally good. Grab a liter of San Miguel beer or a plastic jug of Sangria at a small grocery store and locate a sandwich shop. Ignore the cramped bars; instead join a random marching-band or impromptu parade. Also, wear durable shoes and expect them to be destroyed by the end of Fiesta. Public-male-urination and breaking bottles are Spain's second and third most popular pastimes during the fiesta.
NOTE: If you are taken in by a group of beautiful Pamplonicas (women of Pamplona) that are buying you drinks and dancing with you in the street. Congratulations! They are not going to have sex with you. The women of Pamplona are extremely religious and chaste, so just enjoy yourself while it lasts.