From June 1803, the presence of Napoleon's Grande Armée on the French side of the Channel, notably around Boulogne, provoked very real fears of invasion in England until the summer of 1805. Napoleon was believed to have at around 150,000 men at his command, and a fleet of some 2000 invasion barges. This was the period when many Martello towers were built around the coast to enable sentries to spot any invasion fleet.
Editors. "England fears French invasion". The Literary Encyclopedia. First published 12 December 2007
[http://www.litencyc.com/php/stopics.php?rec=true&UID=4359, accessed 26 August 2016.]