Perched dramatically over Peñafiel, this castle houses the state-of-the-art Museo Provincial del Vino. A comprehensive story of the region's wines, this wonderful museum has interactive displays, dioramas and wine-tasting (book ahead). The rest of the castle, one of Spain's narrowest, is visited on a 40-minute guided tour which explores the castle's crenellated walls and towers (stretching over 200m, they're little more than 20m across). They were built and modified over 400 years, from the 10th century onward.
If you're keen to sample the pleasures of the local product, there are two options. From Tuesday to Friday, you can taste three wines (€6.60) at 1pm and 6pm from April to September, or 1pm and 5pm from October to March. If you'd like the guidance of a sommelier, you can taste four wines (€9.20) at the same times but on Saturdays or Sundays.
