In the Southwestern United States, the term Pueblo refers to communities of Native Americans, both in the present and in ancient times. The first Spanish explorers of the Southwest used this term to describe the communities housed in apartment structures built of stone, adobe mud, and other local material. These structures were usually multi-storied buildings surrounding an open plaza. The rooms were accessible only through ladders lowered by the inhabitants, thus protecting them from break-ins and unwanted guests. Larger pueblos were occupied by hundreds to thousands of Pueblo people. Various federally recognized tribes have traditionally resided in pueblos of such design.