This highly controversial building -- built on the site of Santa María de la Almudena, which in turn occupied the site of Madrid's first Muslim mosque -- must be one of the longest-delayed projects in modern times: 110 years from inception to conception, in fact. Work began in 1883, following a neo-Gothic plan by the Marqués de Cubas. The first thing to be completed was the crypt, which today still retains the 16th-century image of Madrid's patroness, the Virgen de la Almudena. After that, progress was halted until 1944, when new architect Fernando Chueca took over, introducing a neoclassical style. It was eventually finished in 1993 and graced with a visit from the Pope. The bright interior reflects an uncertain blend of hybrid styles and its stained-glass windows are of the "pop art" variety, recently revealed to have been copied. (In defense, their creator claims they are "a vision from God.") The building hit the headlines back in 2004 with the sumptuous wedding of Prince Felipe and Doña Letizia (a former newscaster), the first such royal event in nearly a century, but since then (to date) no similarly eye-catching ceremonies have taken place. You can also pay a visit to the (more interesting) neo-Romanesque crypt to see its well-preserved 16th-century image of the Virgin.