One of these is at Sandy Bay on the exact opposite side of the island from Jamestown, where the Sandy Bay Lines were constructed in the mid 18th century. Below is a plan from 1825.
As you can see the lines originally blocked off the two possible landing points in this area: at Sandy Bay Valley (the larger of the two) and Broad Gut, separated by a rocky promontory (Beach Hill). Here is a photo of the lines from 1863.
The fortification and parade ground on the Sandy Bay Valley side has now been completely removed and/or washed away, but the small section at Broad Gut (in the background of the photo) is still there and in quite a good state of repair. Here are some pictures I took of this when we visited.
You can see the two gun positions in the top of the wall and at the far end an archway through which a stream flows into Sandy Bay.
The second photo is looking down from Beach Hill, where there is a (recently constructed) ‘battery’ with gun, probably a 12pdr smoothbore.
Given the history of this remote island an attempt by loyal French forces to land somewhere, bypass the defences and rescue Napoleon from Longwood, before whisking him off to Brazil or elsewhere, might make an interesting skirmish mini-campaign. And an excellent opportunity to use the Fire & Steel skirmish rules available from The Keep, should I ever get around to it.