The discovery was made in a tunnel that runs under the fort's walls, known as a sallyport, which leads from the inner fort right out to the moat.
The team were reopening old areas of the fort, like this tunnel, for future public use when they discovered a 230-year-old spiral staircase.
"It is cut from stone and seems to floats like something out of Harry Potter.
"Being on the east side of the fort it is highly likely this was part of the first fort built on the island in 1790, explaining it being the only one.
"The work to build the much larger present fort which was started in 1804 swallowed up that first fort and made the sallyport and staircase redundant, so it was blocked up and hidden to this day," the social media posted stated.
"Despite some of the staff being here since the 1980's no one including our heritage team knew of the staircase," the post continued.
The team also revealed they found large animal bones and very old half-full bottles of wine.
Staff said they cannot wait to show off the spiral staircase to visitors.
"We cannot wait to show it to our visitors in time and we thank anyone who has been to us for your support, which allows us to keep discovering areas like this," they said.