Settlement hierarchies appeared on the Southeast Asian mainland around the middle of the first millennium BC, at approximately the same time as iron technology. Insufficient archaeological research has been done in insular Southeast Asia to date this phenomenon. By 2000 BP an interaction sphere linked settlements on the mainland and island sectors of the region. In the early first millennium AD, two forms of urbanization appeared in Southeast Asia: administrative centers in agrarian areas with monumental religious architecture, and trading centers in the lowlands. During the next 1500 years a wide range of urban patterns developed. Densely populated, tightly circumscribed settlements with a wide range of occupations and a monetized economy evolved in the early second millennium, during a period when Chinese merchants began to form enclaves of settlement. From sites of seasonal occupation, these evolved into permanent multiethnic urban places.