 Fresh fears of invasion after 1544 prompted further work being carried out on the blockhouse by Sir Richard Lee , a prominent military engineer , although peace was declared the following year . It is uncertain exactly what shape the fort took ; archaeological investigations suggest that it was probably D @-@ shaped , two storeys tall , with a circular bastion facing the river ; there would have been battery positions on either side of the main defences , and a more modern , angular bastion was added later on the landwards side . By 1546 the King 's accountants estimated that £ 1 @,@ 072 had been spent on building and developing the fortification . 
