Curious little brick tower

This little brick tower is in a corner of St Mary's Rest Garden. There is a nice black and white metal sign headed 'Curious little brick tower' which tells you all about it:

'This is the curious little brick tower. It is believed that the tower was originally an access shaft to the beck culvert tunnel during its construction, though it later became a ventilation shaft for the culvert, foul air being carried up the shaft and out through the grille at the top'.