Book excerpt: From the beginning of the 2nd millennium AD northern India began to fall under the sway of a number of Muslim-Turkic rulers who, at the start of the 13th century, founded the series of dynasties known to history as the Delhi Sultanate. For three centuries these sultans expanded their territory, which led to a dramatic rise in the number of fortifications throughout the subcontinent. This period is the defining age of the Indian castle and the combined influence of the Islamic and Hindu architectural tradition lends these fortifications a unique style. This book covers all the major sites of the period including the fabled seven medieval cities on the site of the present-day city of Delhi.