 The fortifications were strategically placed . London and the newly constructed royal dockyards of Deptford and Woolwich were vulnerable to seaborne attacks arriving up the Thames estuary , which was then a major maritime route ; 80 percent of England 's exports passed through it . The village of Milton and the adjacent town of Gravesend , only 500 metres ( 1 @,@ 600 ft ) apart , formed a particularly important communications point along the river . They formed the centre of the " Long Ferry " traffic of passengers into the capital , and for the " Cross Ferry " over the river to Tilbury , resulting in the local riverbank becoming lined with wharfs . This was also the first point that an invasion force would be able to easily disembark along the Thames , as before this point the mudflats along the sides of the estuary would have made landings difficult . 
