Man has always been a builder. The kind of house he built depended upon the climate, upon his enemies, and upon the building materials at hand. The first houses in many parts of the world were made of wood, for in those days the greater part of the Earth was covered with forests. In other regions the most convenient building material was stone. Men began building houses out of stone very long ago. Although they were built without cement, the remains of a few of them still exist. The ancient Egyptians built very simple houses, by present standards. Having The House. dried the bricks in the sun, they put up four walls, and above these they placed a flat roof. The roof was flat because there was very little rain in Egypt. Although their buildings were simple in construction, the Egyptian art of building was very beautiful. In ancient Russia architecture flourished for the first time in Kiev Russ. The churches of that time were strong buildings with thick walls and small windows. Buildings of the 19-th century are characterized by the use of new materials and by a great diversity of architectural styles. From the end of the 18-th century iron and The House. steel became widely used as alternatives to wood. The 20-th century is notable for widespread use of steel - reinforced concrete. Architecture of the 20-th century is characterized by very high buildings - particularly skyscrapers - and by great diversity of styles which completely differ from those of the past.