Great St Mary's is the official university church of Cambridge, standing directly on King's Parade opposite the Senate House. A church has occupied this site since the 13th century, though the current building dates from the late 15th. Graduation ceremonies are held in the Senate House directly opposite; processions and university events regularly pass through Great St Mary's.
The tower is open to visitors and provides the best unobstructed views of central Cambridge — over the Market Square, across the college rooftops, and down toward King's Chapel. Climbing it costs a few pounds and is worth it on a clear day. The church interior is also open free of charge and has some good 16th-century features.