One of Amsterdam’s best-kept historical secrets is tucked away on Oudezijds Voorburgwal. Following the Alteration and the sacking of all Dutch Catholic churches in 1578, practicing Roman Catholicism was banned, so the Catholics had to find ways to worship in secret. Between 1661 and 1663, the wealthy Catholic merchant Jan Hartman bought this stately town house and two others behind it, converting all three attics into a clandestine but lushly decorated Catholic chapel. Worshipers entered from a side street and climbed the narrow stairs to the hidden third-floor chapel, which could accommodate a congregation of 150. The secret chapel is resplendent with its Baroque flourishes, organ, marble columns, and oil paintings behind the altar once more shining like new. A modern visitor’s center in the adjoining house features an unremarkable collection of religious artifacts that you can quickly breeze through on your way to the main attraction.