Located behind Porto's City Hall, this 19th-century church is a gem of Neoclassical style. Even though it is not a popular tourist attraction, it is definitely worth visiting for its interior decorations including sculptures and exquisite woodwork. Due to its acoustics, many concerts take place here throughout the year.
Wikipedia (Portuguese) https://pt.wikipedia.org/wiki/Igreja_da_Trindade_(Porto)
Address 19 Rua da Trindade, Porto 4000541, Portugal
Coordinates 41°9'3.88" N -8°36'37.099" E