Pendennis is one of the first English forts to be designed specifically for cannon, and in consequence is utterly unlike a mediaeval castle in its design.It is low to the ground,
but with several gundeck levels, and planned to enable its cannon to have a broad sweep over the estuary and the sea.
A smaller but similar castle was also built at St.Mawes, on the opposite bank of the entrance to the estuary.
As it happened, the Spanish Armada of 1588 sailed straight past all Henry VIII's castles and did not attempt any landing on the south coast, which, in retrospect, might have been a more effective strategy. But the danger from Spain was present for the rest of Elizabeth's reign, and Pendennis was surrounded with a pentagonal fortification in 1597.
Pendennis was never attacked by a foreign invader. The castle only saw action when it endured a four-year siege in the Civil War, terminated when its Royalist garrison was obliged to surrender in 1646. It was rebuilt and modernised in the 18th century, and formed part of the coastal defences in both World Wars, when anti-submarine mines were laid across the estuary. Until 1957 it was used for artillery training.
The site is now administered by English Heritage, and includes a later barracks and a museum.
Although Falmouth is now a major port, it was not built as a town until 1613. The earlier town was Penryn, a short distance upriver. Both Penryn and St. Mawes became classic "rotten boroughs", continuing to elect two Members of Parliament apiece, despite their very few inhabitants, until the Great Reform Act of 1832.