Impel Down, the world's largest prison.
It was called an impregnable fortress, not because its walls were especially thick, nor because it lay in the deserted Calm Belt, where Sea Kings were abundant.
Impel Down was known as the world's largest prison and hailed as an impregnable fortress.
The biggest reason was its location: it was extremely close to Marineford, the headquarters of the Marine Headquarters, and Mary Geoise, the headquarters of the World Government.
Because of its geographical location, once anything major happened at Impel Down, the powerful figures of Marineford and Mary Geoise could rush over to support it very quickly.
And for that very reason, the experts of the seas did not dare attack Impel Down.
Because touching Impel Down was equivalent to directly challenging the world's largest organization and the World's Number One Violent Organization.
Thus, Impel Down's impregnable fortress referred to Marine Headquarters and Mary Geoise.
Although Impel Down's own guards were also extremely powerful, they were not enough to make the true top-tier fighters of this world afraid to touch it.
Golden Lion's escape back then, as well as Luffy and the others breaching Impel Down, were the best proof of that.
Once Marine Headquarters or Mary Geoise was held back by some force,
then against overwhelmingly powerful fighters on the level of Golden Lion and the others, Impel Down, the world's largest prison, had little ability to resist.
And it was precisely for this reason that Roy had Lucci secretly push Spandine into acting quickly.
With most of Marine Headquarters' and the World Government's forces now drawn to the North Blue, Impel Down might still appear to be an impregnable fortress on the surface, but having lost the deterrence of Marine Headquarters and Mary Geoise, it had actually entered its most dangerous moment.