The giant underwater prison Impel Down has always been one of the most interesting places in the story however the construction of this place has been somewhat of a mystery with most assuming it was built by fish-man slaves.
However with the revelation in the recent chapters it is likely that this isn’t the case and was instead a large tower which was built on the surface Pre-Void Century which was later submerged, however if this was the case the building being a prison is quite odd as in the current story it makes sense due to the advantage of it being underwater and hard to break into but in the past it would have been much more effective to build a flat complex as a prison.
This leads me to my main point in that this was not originally designed to be a prison and was instead built as a large castle or fort and that after being mostly submerged was converted in to what it is today replacing most of the inside with cells and torture devices.
It’s likely this castles previous owners either died or left the place after the events of the Void Century with it being possible that if these owners did somehow survive that they would want to move their royalty to a more geographically advantageous spot such as on the Red Line far above where the waves could touch them potential making impel down the World Government’s original Headquarters prior to taking over the red line and them reusing this building in place of making a building for their prison.
*Theory by Ok-Regular-959
