The establishment of the Dutch East India Company in the early 17th century lead to a quick increase in the volume of the slave trade in the region; there were perhaps up to slaves in various Dutch colonies during the 17th and 18th centuries in the Indian Ocean. For example, some 4000 African slaves were used to build the Colombo fortress in Dutch Ceylon. Bali and neighbouring islands supplied regional networks with slaves 1620–1830. Indian and Chinese slave traders supplied Dutch Indonesia with perhaps slaves during the 17th and 18th centuries.