Because the crossing is open on four sides, the weight of the tower or dome rests heavily on the corners; a stable construction thus required great skill on the part of the builders. In centuries past, it was not uncommon for overambitious crossing towers to collapse. Sacrist Alan of Walsingham's octagon, built between 1322 and 1328 after the collapse of Ely's nave crossing on 22 February 1322, is  the "... greatest individual achievement of architectural genius at Ely Cathedral" according to architectural historian Nikolaus Pevsner.
Because the crossing is open on all sides, the weight of the tower or dome rests heavily on the corners; a stable construction thus demanded great skill from the builders. In past centuries, it was not uncommon for ambitious crossing towers to collapse. Sacrist Alan of Walsingham's octagon, built between 1322 and 1328 after the collapse of Ely's nave crossing on February 22, 1322, is  the "... greatest individual achievement of architectural genius at Ely Cathedral" according to architectural historian Nikolaus Pevsner.