Both apses are flanked by paired towers. Many of the smaller towers are circular, as at Worms Cathedral. There may be numerous towers of varied shapes and sizes. The crossing is generally surmounted by an octagonal tower, as at Speyer Cathedral. Spires are of roofed timber rather than stone and take a variety of forms, the most distinctive being the Rhenish helm. Stone is sometimes used for Rhenish helms as at the eastern end of the Basilica of Our Lady, Maastricht. The towers and apse of the western end are often incorporated into a multi-storey westwerk. These latter take a great variety of forms, from a flat façade as at Limburg Cathedral, a flat façade with projecting apse at St Gertrude, Nivelles and a rectangular projecting structure of several storeys that juts beyond the towers as at St Serviatius, Maastricht. The transepts do not project strongly. In the Rhineland, the exterior walls and towers are encircled with courses, Lombard bands and dwarf galleries, which serve to emphasise the individual mass of each component part of the whole, as at Speyer Cathedral. Wheel windows, ocular windows and windows with simple quatrefoil tracery often occur in apses, as at Worms Cathedral. Wooden roofs were common, with an ancient painted ceiling retained at St Michael's, Hildesheim. Stone vaults were used at a later date than in France, occurring over the aisles at Speyer in about 1060. Notable examples Aachen Cathedral, Aachen, North Rhine-Westphalia, Germany (Carolingian) Gernrode Abbey, Gernrode, Saxony-Anhalt, Germany St. Michael's Church, Hildesheim, Lower Saxony, Germany (Ottonian) Hildesheim Cathedral, Hildesheim, Lower Saxony, Germany Brunswick Cathedral, Braunschweig, Lower Saxony, Germany Speyer Cathedral, Speyer, Rhineland-Palatinate, Germany Worms Cathedral, Worms, Rhineland-Palatinate, Germany Mainz Cathedral, Mainz, Rhineland-Palatinate, Germany Naumburg Cathedral, Naumburg, Saxony-Anhalt, Germany Trier Cathedral, Trier, Rhineland-Palatinate, Germany Laach Abbey, Andernach, Rhineland-Palatinate, Germany Bamberg Cathedral, Bamberg, Bavaria, Germany Limburg Cathedral, Limburg, Hesse, Germany Collegiate Church of Saint Gertrude, Nivelles, Wallonia, Belgium Collegiate Church of St. Bartholomew, Liège, Wallonia, Belgium Tournai Cathedral, Tournai, Wallonia, Belgium Basilica of Our Lady, Maastricht, Limburg, the Netherlands Basilica of Saint Servatius, Maastricht, Limburg, the Netherlands Munsterkerk, Roermond, Limburg, the Netherlands Susteren Abbey, Susteren, Limburg, the Netherlands Romanesque churches in Scandinavia Influences Norway, Sweden and Denmark were separate kingdoms for much of the period. Much of Norway was united from the late 9th century until 1387 under Harold I and his successors. Cnut the Great briefly united Denmark, England, Norway and parts