The greatest period of Inca expansion occured during the reigns of Pachacuti (1438–71), Tupa Inca (1471–93), and Huayna Capac (1493–1527). From the mountain stronghold of Cuzco, they subjugated the surrounding kingdoms and territories, absorbing their civilizations and their peoples. By 1525, they dominated much of the west of the continent, relying on fortified strongholds, an extensive system of roads and bridges, and obligatory military service to control local populations. This title takes a detailed look at the development of Incan fortification techniques, and examines how they came to be overrun by the Spanish conquistadors.