St Paul's Cathedral familiar with the unique octagonal lantern tower over the crossing of Ely Cathedral, which spans the aisles as well as the central nave, unlike the central towers and domes of most churches. Wren adapted this characteristic in designing the dome of St Paul's. In section St Paul's also maintains a medieval form, having the aisles much lower than the nave, and a defined clerestory. The most notable exterior feature is the dome, which rises to the cross at its summit, and dominates views of the City. The height of 365 feet is explained by Wren's interest in astronomy. Until the
Ely Cathedral weight from the octagonal tower, four new arches were added at the west crossing to strengthen the tower. The extra weight of these works may have added to the problem, as at the end of the fifteenth century the north-west transept collapsed. A great sloping mass of masonry was built to buttress the remaining walls, which remain in their broken-off state on the north side of the tower. The Galilee Porch is now the principal entrance into the Cathedral for visitors. Its original liturgical functions are unclear, but its location at the west end meant it may have been used