This is part of a series of questions on the Gemara in Bechoros 8b-9a. Below is a summary of the relevant parts of the Gemara. After being challenged by Caesar, R' Yehoshua went to Athens to defeat the Athenian Elders in a battle of wits.
When R' Yehoshua arrived in Athens, he found a butcher slaughtering an animal. R' Yehoshua asked to buy his head. The butcher, understanding him to mean the head of the animal, agreed, for half a zuz. R' Yehoshua responded that he meant the head of the butcher, but he would let him go if he would take R' Yehoshua to the Elders.
The butcher was terrified. "They'll kill me if I reveal their location!" he cried.
R' Yehoshua came up with a plan. "Take your load around the city. Put it down outside their chambers, as if you need to rest, and at that point I'll enter. They won't know what hit them." And so it was. The butcher thus led R' Yehoshua to the Elders.
Now, there were guards by the doors on the outside, and there were also guards on the inside of the entrance hall. The ones on the outside were to prevent people from going inside the main building, while the ones on the inside were to prevent the Elders from leaving. If footprints were seen going in, the Elders would kill the outer guards; if they were seen going out, the Elders would kill the inner guards. Thus, R' Yehoshua put just one of his shoes on backwards and walked in. Seeing contradictory footprints, the Elders killed all of the guards.
Now, if nobody was allowed to reveal the Elders' location, presumably the reason is that nobody unauthorized was allowed to enter. So why did the guards let R' Yehoshua in?