are on the western side and rise to a height of 37 m. They form the distinctive "" of the west front. Such twin-tower façades are common in the Rheinland, for example at Xanten, and Koblenz. The pointed crossing spire stands above all the other spires with a height of 66 m at the centre of the building. This height is the result of a lightning strike in 1774, before which the tower was 6.5 metres higher. The corner towers on the southern transept were erected in 1863. The west front is divided into five levels. The most eye-catching stylistic element is a huge round window, surrounded by eight small rosettes, which forms a clear centre of the west front. The rosette symbolises the four Evangelists. Despite the symmetry of the twin towers, there is rich variation in forms and building elements, e.g. round and pointed arches, pilaster strips, small pillars, archivolts, windows and blind arches. The upper level of the north tower includes Gothic elements (e.g. window tracery). During restorations between 1872 and 1873, the polychrome exterior painting of the cathedral was removed (the colours were, previously, white, red, yellow-brown, black and a little green) and the stone was left bare. Between 1968 and 1972, the polychrome exterior was restored, using remains of the colour from the period before 1872 in order to reconstruct the old patterns. Interior The interior of the cathedral (nave and choir) is dominated by the completely undecorated buttresses, which reach all the way to the ceiling. More of the buttress system is hidden in the galleries of the side-aisles. The comparatively plain and bright interior is marked by a narrow, high central nave. It is divided into four levels with arcades, galleries, triforia and clerestories. Organ Organ playing is repeatedly documented in the 14th century; in 1443 two organs are mentioned. After several renovations in the 16th and 17th centuries, the main organ was broken in 1749 and created a new organ (1750–1752). A. and M. Keller of Limburg converted the Baroque housing to a neo-Romanesque style between 1872 and 1877, while retaining Köhler's register. An almost complete rebuild within the neo-Romanesque casing was carried out in 1912 by Johannes Klais, a thorough renovation in 1935 and a neo-Baroque extension in the neo-Baroque style in 1960, by the same organ builder. The current organ was installed in a modern casing in the west gallery