The Túnel del Castell is a network of underground passages beneath the castle hill, originally constructed as an air-raid shelter during the Spanish Civil War (1936–39). Dénia was a Republican stronghold during the war, and the tunnels were dug to protect the civilian population from Nationalist bombing raids. The shelter could hold hundreds of people and was equipped with medical facilities and food stores.
Guided tours take you through the tunnels and explain both the construction and the history of the town during the war years. The tunnels connect with the castle complex above and give a very different perspective on the hill's layers of history — from Moorish fortress to 20th-century shelter in a few minutes of walking. Tours run in Spanish and Valencian; check locally for English-language options.