and much of the architecture of the 20th century. The International style was led by groups like GATEPAC. Spain is currently experiencing a revolution in contemporary architecture and Spanish architects like Rafael Moneo, Santiago Calatrava, Ricardo Bofill as well as many others have gained worldwide renown. Many architectural sites in Spain, and even portions of cities, have been designated World Heritage Sites by UNESCO. Spain has the third highest number of World Heritage Sites in the world; only Italy and China have more. These are listed at List of World Heritage Sites in Europe: Spain. Prehistory Megalithic architecture In the Stone Age, the most common megalith found in the Iberian Peninsula was the dolmen. The plans of these funerary chambers used to be pseudocircles or trapezoids, formed by huge stones stuck on the ground, and others over them, forming the roof. As the typology evolved, an entrance corridor appeared, and gradually took prominence and became almost as wide as the chamber. Roofed corridors and false domes were common in the most advanced stage. The complex of Antequera contains the largest dolmens in Europe. The best preserved, the Cueva de Menga, is twenty-five metres deep and four metres high, and was built with thirty-two megaliths. The best preserved examples of architecture from the Bronze Age are located in the Balearic Islands, where three kinds of construction appeared: the T-shaped taula, the talayot and the naveta. The talayots were troncoconical or troncopiramidal defensive towers. They used to have a central pillar. The navetas, were constructions made of great stones and their shape was similar to a ship hull. Iberian and Celtic architecture The most characteristic constructions of the Celts were the castros, walled villages usually on the top of hills or mountains. They were developed at the areas occupied by the Celts in the Douro valley (Portugal) and in Galicia. Examples include Las Cogotas, in Ávila, the Castro of Santa Tecla, in Pontevedra in Spain. The houses inside the castros are about 3.5 to 5 meters long, mostly circular with some rectangular, stone-made and with thatch roofs which rested on a wood column in the centre of the building. Their streets are somewhat regular, suggesting some form of central organization. The towns built by the Arévacos were related to Iberian culture, and some of them reached notable urban development like Numantia. Others were more primitive and usually excavated into the rock, like