The original use of a sally point, of course, was to make sallies when your castle was besieged. Your attackers could gather in the point and sally forth without any danger that anyone would get through the closed inner door behind them. Then, when they withdrew, they could close the outer door, and trap any besiegers with them and the whole might of the castle.
It merely happens to be identical to a good way to transfer prisoners.
The original use of a sally point, of course, was to make sallies when your castle was besieged. Your attackers could gather in the point and sally forth without any danger that anyone would get through the closed inner door behind them. Then, when they withdrew, they could close the outer door, and trap any besiegers with them and the whole might of the castle.
It merely happens to be identical to a good way to transfer prisoners.
it’s very effective for traffic control in both directions!
For full pedantry points, I will note that it is actually a sally *port*
https://en.wikipedia.org/wiki/Sally_port