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
Porth-y-Tŵr and rooms adjoyning to it where the Bishop's & Mayor's courts were held". Porth-y-Tŵr was in ruins by the 20th century and, in 1929, the octagonal tower was substantially restored by ecclesiastical architect W. D. Caroe, with funding coming from an anonymous donor. In the 1930s a ring of bells were installed in the octagonal tower – the bells in the tower of St Davids Cathedral had been removed in 1730 to prevent the cathedral's tower collapsing. Two further bells were added in 2001, donated by the American Friends of St Davids Cathedral. There are ten bells in total, ranging