Until the 18th century, Bombay consisted of seven islands separated by shallow sea. These seven islands were part of a larger archipelago in the Arabian sea, off the western coast of India. The date of city's founding is unclear—historians trace back urban settlement to the late 17th century after the British secured the seven islands from the Portuguese to establish a secure base in the region. The islands provided the British with a sheltered harbour for trade, in addition to a relatively sequestered location that reduced the chances of land-based attacks. Over the next two centuries, the British dominated the region, first securing the archipelago from the Portuguese, and later defeating the Marathas to secure the hinterland.