Wandering around the interior of the peninsula known as The Garrison, the most impressive thing is the size. Why on earth did they feel the need to enclose such a vast area, and how would they ever man it effectively?
Fortifications started in 1593 with the Star Castle. Shortly thereafter work commenced on the landward stretch of the walls between Well Battery and the Bentham Batteries, overlooking the growing settlement of Hugh Town.
During the Civil War an earthwork bank was built all the way round the headland, a surviving part of which can be seen near to Morning Point Battery.
Work on a more substantial wall around the seaward side of the headland started in 1741, and progressed in a clockwise direction until it reached Steval Point in around 1748. Then the War of the Austrian Succession ended, the threat of a French invasion disappeared, and building work stopped. The coast between here and King Charles' Battery was thus left to its own devices.
Two large gun batteries were built on the top of the hill in 1898-1901, along with some searchlight positions and a quick firing battery at Steval Point. There are also a number of remains from the Second World War.
As you go around keep an eye open for the black rabbits, introduced during the 19th century by Augustus Smith of Tresco Abbey Gardens fame. There seem to be quite a few.
The Garrison Gate bears the date 1742 and the initials of Abraham Tovey, the 'Storekeeper and Master Gunner', who oversaw the construction of the garrison walls and the rebuilding of the gate.
The arrogance and ambition of a man who chose to place his initials immediately below those of King George and Francis Godolphin (the Governor of the islands), probably goes a long way to explain the huge area he attempted to enclose.
After all, to man it effectively he would need far more than the six gunners he had under his command.
On either side of the gate are two earlier 17th century buildings that were once barracks but are now holiday lets.
The need to protect the Isle of Scilly was brought into sharp focus following Elizabeth I decision in 1585 to support the Netherlands in their struggle for independence from Spain.
The Spanish Armarda of 1588 showed that there was a very real threat of invasion and plans were immediately drawn up to fortify St Mary's.
Rather than complete the unfinished fort known as Harry's Walls, the decision was taken to build the Star Castle, which was completed in 1593 at a cost of £958 11s 2d. It is now a privately owned 4-star hotel.
King Charles' Battery
Little is known of the early history of King Charles' Battery.
Its walls are made from a mixture of stone and sod-work which implies that predates the rebuilding of the walls in the 1740s.
It also appears on a map produced by Christian Lilly in 1715. It is, therefore, likely to date from the Civil War period.
In 1793, during the Napoleonic Wars, the Garrison was rearmed, including the provision here of a traversing platform allowing one of the guns to be swiftly rotated.
The Woolpack Battery was the largest of the Garrison gun batteries. It was designed to prevent ships from approaching the harbour via St Mary's Sound, the route taken to this day by the Scillonian III and other large vessels at low water.
Dating from the 1740s, during the Napoleonic Wars it was armed with eleven long nine-pounder guns. However they were never fired in anger.
These days it is home to a collection of short nine-pounders and 32-pounders dating from around 1800.
The 1901 Woolpack Battery
As I discuss at interminable length in my article on the 100 Ton Gun in Gibraltar, in 1884 Sir Andrew Noble invented Smokeless Gun Powder, a significantly more powerful form of powder which rendered all existing guns redundant.
A large number of new defences on St Mary's were, therefore, built in the period 1898 to 1901 to accommodate the new technology. These included a pair of gun batteries on the top of the Garrison (called Woolpack and Steval confusingly enough) together with a small barracks block between them.
Steval is now used by a gun club, and the barracks is holiday accommodation. Neither can be easily seen by the general public. Woolpack, on the other hand, has a public path around the perimeter but, as it is used to provide accommodation for volunteers working for the Isles of Scilly Wildlife Trust, it is not possible to enter the site.