This is the Ruins of St. Paul’s, a 17th century Portuguese church dedicated to St. Paul the apostle. I visited this place when I visited Macau, China earlier this week. According to Wikipedia, this church was built by the Jesuits and was one of the largest Catholic churches in Asia at the time. However, it was destroyed by a fire during a typhoon in 1835 and this wall is all that it remains of the once well-known church.
This wall is part of the Fortress of Our Lady of the Mount of St. Paul, which is the historical military center of Macau. This fortress was also built in the 17th century and was used to protect the properties of the Jesuits in Macau. This remained a restricted military area until 1965 when it was convert into a weather observatory and was opened to the public.
If you look carefully above, you will notice the ends of the cannons sticking out of the walls.