Storkyrkan ("The Great Church") was first mentioned in writing in 1279, and was "purified from Papist superstitions" (as the plaque on the east side explains) in 1527 thanks to the zeal of king Gustav I. Parts of the same east side was chopped off in the aforementioned process in order to give the aforementioned king's cannon a clear field of fire from the castle, should the recetly-defeated Danish king return along the Källargränd alley.
The church houses Vädersolstavlan ("The Sun Dog Painting"), depicting a halo phenomenon that was seen over Stockholm in 1535. It also depicts the church as being larger than the castle, something which the king (Gustav I again) saw as treason and had the clergyman responsible for commissioning the painting sentenced to death. (He was later pardoned.)
Swedish monarchs were traditionally crowned here, until Gustav V decided to skip that step and get right on with ruling as king.