Harry's Walls are the remains of an unfinished artillery fort, started in 1551 by the government of Edward VI to defend the island of St Mary's in the Isles of Scilly.

Constructed to defend the harbour of Hugh Town from possible French attack, the fortification incorporated Italianate-style bastions with protective orillons and would have been the most advanced design in the kingdom at the time.

[4] The Duke fell from power in 1549 and a fresh survey was conducted by Captain William Tyrrell; work then began on protecting the island of St Mary's.

[5] In May 1551, John Killigrew, the captain of Pendennis Castle in Falmouth, was ordered to construct a fort on St Mary's, probably with the intent of guarding the entrance to the new harbour at Hugh Town.

[8] A document from the early 1550s noted that a brewhouse and a mill were ready to be sent to St Mary's from South Wales as part of the project, to be installed by the fresh water pond.

[11] It was intended to develop the Italianate ideas first introduced in Portsmouth and Yarmouth under Henry, and would have been what the historian Andrew Saunders considers to be "the most advanced piece of military engineering for its date to be seen in this country".

[19] Instead, the high costs of England's wider fortification programme - Edward's government spent an unsustainable £35,228 on construction work during the five years of his reign - are blamed for making its completion unviable, particularly once the French invasion threat had passed.