The ruins of the Igreja do Carmo (Church of the Carmelites) are a reminder of the devastation caused by the earthquake of 1755, which destroyed most of downtown Lisbon. Inside the ruins is a small archaeological museum.Nuno Álvares Pereira - a general who was instrumental in helping king John repel the Castilian troops - vowed to build the world's most beautiful church if he would defeat the Castilians in 1385 at the battle of Aljubarrota. On the first of November 1755 an earthquake measuring nine on the Richter scale caused the collapse of numerous buildings, including twenty churches. The first tremors were followed by fires. Many of the surviving inhabitants of Lisbon fled to the open area near the Tagus river, but an hour after the first tremor, they were engulfed by twenty meter (66ft) high waves. One of the churches that collapsed during the devastating earthquake was the Carmo church, which at the time was filled with All Saints Day worshippers. The roof of the building collapsed on top of the worshippers and only the walls and arches kept standing.