High Gate Land
Access to the plateau atop the Land is by elevator of the rope-and-wooden platform variety, powered by Pones. This makes the Land impregnable by conventional warfare, but not to subterfuge.
On the plateau surface are raised letters spelling out ON EPEN NY. Apparently there are letters on the underside of the Land, from reports of explorers in the Underlay.
High Gate Land is a Britsh One Penny coin, landed tails-up on the carpet. The "tails" side of a penny shows a portcullis - a gate on the highest face of the coin. On the heads side are the letters spelling ELIZABETH II - the Queen who appears on all post-decimalization British coins.
Interestingly, given the history of the book, the writing on the coin, "ONE PENNY" was introduced in 1982, before which it read "NEW PENNY".
Given the Vortgorns have not found steel, either they have not burrowed far enough, or the coin is pre-1992 when they were solid bronze instead of plated steel. (Most likely the latter as the book was published in 1971)