The history of Liverpool can be traced back to 1190 when the place was known as 'Liuerpul', possibly meaning a pool or creek with muddy water, though other origins of the name have been suggested. The borough was founded by royal charter in 1207, but Liverpool remained a small settlement until its trade with Ireland and coastal parts of England and Wales was overtaken by trade with Africa and the West Indies, which included the slave trade. By the start of the nineteenth century, a large volume of trade was passing through Liverpool. The population grew rapidly, especially with Irish migrants; by 1851, one quarter of the city's population was Irish-born. Although a small motte and bailey castle had earlier been built by the Normans at West Derby, the origins of the city of Liverpool are usually dated from 28 August 1207, when letters patent were issued by King John advertising the establishment of a new borough, "Livpul", and inviting settlers to come and take up holdings there. Initially it served as a dispatch point for troops sent to Ireland, soon after the building around 1235 of Liverpool Castle, which was removed in 1726. In the middle of the 16th century the population of Liverpool had fallen to around 600, and the port was regarded as subordinate to Chester until the 1650s. In 1571 the people of Liverpool sent a memorial to Queen Elizabeth, praying relief from a subsidy which they thought themselves unable to bear, wherein they styled themselves "her majesty's poor decayed town of Liverpool. By the end of the sixteenth century, the town began to be able to take advantage of economic revival and the silting of the River Dee to win trade, mainly from Chester, to Ireland, the Isle of Man and elsewhere. In June 1644 Prince Rupert of the Rhine arrived in Liverpool with 10,000 men in an attempt to capture Liverpool Castle. A sixteen-day siege of Liverpool then took place. From that time may be traced the rapid progress of population and commerce, until Liverpool had become the second metropolis of Great Britain. In 1699 Liverpool was made a parish on its own by Act of Parliament, separate from that of Walton-on-the-Hill, with two parish churches. On 3 October 1699, the very same year that Liverpool had been granted status as an independent parish, Liverpool's first 'recorded' slave ship, named Liverpool Merchant, set sail for Africa, arriving in Barbados with a 'cargo' of 220 Africans, returning to Liverpool on 18 September 1700. The first wet dock in Britain was built in Liverpool and completed in 1715. By the close of the 18th century 40% of the world's, and 80% of Britain's Atlantic slave activity was accounted for by slave ships that voyaged from the docks at Liverpool.