including some by Philipp Veit, were painted to decorate the inside of the cathedral. 20th century restorations Conservation efforts began in the 1900s to save the cathedral from further damage. After a lowering of the groundwater, the wooden substructures became rotten and the foundations started to fail and needed to be replaced. Beginning in 1909 the old foundations were underpinned. Works stopped in 1916 due to World War I. Between 1924 and 1928 the fundaments were completely reinforced by a new fundament made of concrete. Concrete and steel were used to anchor the towers and main vault. A new floor, made of red marble, was constructed in this period. Architect Paul Meyer-Speer engineered a system to modify the inner walls with colorful sandstone, removing most of the paintings by Veit and restoring a look similar to the original Willigis-Bardo construction. Unfortunately this system did not withstand continuing restoration efforts, and by 1959 most of the color was gone. In World War II, Mainz was a target of Allied bombing multiple times. The cathedral was hit several times in August 1942. Most of the roofs burned, and the top level of the cloister was destroyed. The vault, however, withstood the attacks and remained intact. The damaged elements were restored as authentically as possible, a process which continued well into the 1970s. In addition, much of the glass in the cathedral was replaced. The outside of the cathedral was colored red to match the historical buildings of Mainz. In addition, extensive cleaning and restoration efforts were undertaken, ending in 1975. In that year, the thousandth year since the beginning of the cathedral's construction was celebrated. In 2001, new efforts were begun to restore the cathedral both inside and outside. They were expected to take from ten to fifteen years. In 2004, two large windows, by the renowned glass artist Johannes Schreiter, were installed. Emperors and the cathedral When Mainz was an archbishopric, the cathedral was the official seat of the archdiocese. In 1184 Emperor Barbarossa celebrated the accolade of his sons in the cathedral. The feast of 1184 on the Maaraue, the Diet of Pentecost, became legendary in history as the greatest feast of the Middle Ages. It was from this cathedral that Frederick Barbarossa, the Holy Roman Emperor of the time, officially announced his support for the Third Crusade during the Curia Christi of 27 March 1188. During the Middle Ages,