Victor witnesses Tilbury Fort at the end of volume three, chapter one, on his tour with Clerval. They are making their way through the towns of England in December toward London, where chapter two begins.
Tilbury Fort, near the Thames' mouth, is an incredible example of 17th-century military engineering due to its precise geometric shape. Although never seeing combat, it likely reminds Victor of the Spanish Armada because of Elizabeth I's famous speech here. The Spanish Armada itself was a large Spanish fleet whose defeat was a great military achievement for England.
This location's mention shows both Victor's and, by proxy, Shelley's historical breadth of their tours. This is reflected in the line before, where Victor notes how it seems every town he passes through has some story. It is also another location that was likely associated with English patriotism. Both themes of stories and patriotism can be interpreted as criticisms of Victor making light of the monster's stories and turning away from his friends and family.
Craven, Steven. “Tilbury Fort: The Water Gate,” Geograph, 25 Dec. 2013, www.geograph.org.uk/photo/3789674.
“History of Tilbury Fort.” English Heritage, www.english-heritage.org.uk/visit/places/tilbury-fort/history/.