Public art in Barcelona is a designated group of monuments and outdoor sculptures in the city. The artworks in city's architecture and network of museums, parks, and gardens, put an artistic stamp on the Catalan capital. Public art in the city developed in the 19th century, although the first municipal commission was the 1673 monument to Saint Eulalia in Pedró Square. Artworks are typically located in niches and on the façades of public buildings. They are concentrated because of the city's enclosure by medieval defensive walls. The walls were demolished in the 19th century, sparking a boom in public works such as Ildefons Cerdà's Eixample project. The city annexed several municipalities in the early 20th century, increasing the space available for public art. Some were commissioned for particular events such as the 1888 and 1929 Expositions, the 1992 Summer Olympics and the 2004 Universal Forum of Cultures. Public art in Barcelona is regulated by the city council and protected by Law 9/1993 of Catalan Cultural Heritage, which guarantees the protection, conservation and research of cultural heritage at several levels. Urban planning includes architecture, engineering, gardening, industrial arts and sculpture. Urbanism relates to politics, economics, history, geography and sociology. Monuments are generally located in parks and gardens, squares and intersections, or near public buildings. There are several types; until the 19th century the most common were the column, the triumphal arch and the equestrian statue, three types of monuments inherited from Roman art. Later monuments were busts placed in niches, medallions, fountains, steles and pedestals, friezes, reliefs, commemorative plaques, gravestones, followed by installations, land art and multidisciplinary works. Commonly-used materials include wood, plaster, terracotta and other ceramics, stone, marble, bronze, iron, steel, concrete and aluminium. Themes include homage to a character or event: religious, mythological, symbolic, allegorical, historical, political, military, scientific or artistic.