... In the 16th century, a thick-walled granite house that still stands in a row of narrow buildings along a cobbled street held a dangerous secret. At the back, steep steps lead down to a warren of alleys ideal for conspiratorial comings and goings that helped keep an outlawed religious ceremony hidden.

Four centuries later, the secret of the synagogue is out. The mystery began unravelling when Fr Agostinho Jardim Moreira, a Catholic priest, bought the four-storey house for use as an old people's home for his parish. When construction workers told him they had come across a false wall, he told them to pull it down - sensing a hidden tale.

He had studied the city's Jewish history and knew his parish had been a Jewish quarter in the 15th and 16th centuries. He also knew that, after they were forced to convert to Catholicism in 1496, many Jews privately kept their faith and worshipped in secret. "I suspected that false wall was hiding something," said Fr Moreira. "I knew there had to be some kind of Jewish symbol behind it."

A worker's sledgehammer proved his hunch right. Beyond the wall was a room with a medieval holy ark - a nook in the wall of a synagogue where Torah scrolls are kept. Only two other arks from the period have been found in Portugal...