The Castle of the Holy Cross Has a Secret Underground Mosque - Apparently.
The Castle of the Holy Cross in Barcelona, Spain, was built in the 12th century as a military fortress and royal residence. However, what's lesser known is that it also houses a hidden mosque, built in the 13th century for the Muslim population of the city. This underground mosque, known as the "Aljaferķa", is a rare example of a medieval Islamic architectural feature in a European castle. Archaeological excavations have confirmed its existence, and it remains one of the city's best-kept secrets.