St Nicholas (sv Mikuláš) church, in Malostranké Námestí, is described by the Rough Guide as "easily the most magnificent Baroque building in the city". Certainly it's very splendid inside, gold everywhere but a fairly airy feeling. A Kc70 (about £2.20) entry fee. An interesting exhibition of paintings upstairs in the gallery, with an opportunity for some nice views looking down into the nave.
Also in the gallery a small, self-contained, single-manual organ. I spoke to the attendant who said there are concerts on this organ at 6:00pm daily (I was unable to get to one unfortunately). I think Mozart played the main organ, over the west door, which the attendant implied is not used now. After Mozart's death in 1791, there was a performance in the church of the Requiem by Franz Anton Roesler-Rosetti, with the conductor of the theatre orchestra, Joseph Strobach, directing 120 of 'the best Prague musicians', including Josefina Dušek singing soprano solo. Outside, the church dominates the square and the surrounding buildings. A shame about the recycle bins in front of the main entrance.