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. Renovated in 2012, the secret chapel is once more resplendent with its Baroque flourishes, organ, marble columns, and oil paintings behind the altar once more shining like new. A visitor center is being built in the adjoining house and rooms decorated in period style are opening up on the lower floors to enhance the museum.