Built just over 200 years ago, Dover boasts a 140 feet deep shaft lined with a unique triple staircase, built for the military, and occasionally open to the public.

It’s purpose was simple. The military forts were high up in the hills over Dover, but the sea was, well, at sea level, and getting between the two required a long sloping road that was often muddy and dangerous in the wet.

Much quicker for the soldiers if there was a staircase between the two, and Lieutenant-Colonel William Twiss proposed the Grand Shaft to be dug down through the chalk hills to the sea.

The resulting shaft was 26 feet (7.9 m) in diameter, 140 feet (43 m) deep with a 180 feet (55 m) tunnel connecting the bottom of the shaft with the main road outside.

The use of three staircases winding around the core of the shaft is described in many places as being so that officers and soldiers could be separated, although that seems to be a later invention. Three staircases simply meaning that more people could get up or down at once, and in case of war, speed was essential.

As military requirements changed over the decades, the military forts closed, the shaft was abandoned, and filled with junk, including a dumped car. The shaft was restored in the 1980s, and these days is open once a month by the Western Heights Preservation Society.

I approached from the top of the shaft, being already at that height, and there was little to suggest anything was happening to open it up, but the gates were unlocked, so in to have a look.

A top of the Grand Shaft is itself at the bottom of a steep staircase, but here, you choose which of the three spirals to take — all three are identical, but it could be fun to split up if in a group as you can wave to each other through the windows as you go down.

Realistically, it’s a long winding walk down lots of stairs, peering into the shaft through the windows that were cut in to allow daylight into the stairs — although that’s now supplemented by electric lights.

It’s a bit dank and musty in places, but that adds to the fun of the decent, and it’s not long before you’re at the bottom and peering up all the way to the top.

Then, the long tunnel to the daylight at the far end, and sudden realisation that you probably did the whole thing backwards, as down here is the open-day reception table and volunteers to tell you about the Grand Shaft before you climb up.

Climbing down is much easier though.

The Western Heights Preservation Society currently open the Grand Shaft roughly on the third Saturday of most months — to check their upcoming dates go here.

Entry is free.