User blog comment:Rassilon of Old/Young Justice spoilers at Comic-Con 2012/@comment-5185866-20120716122048/@comment-5185866-20120718035203

The Joker was led into the cave by that other guy, dont remember his name, but he's the current teacher for superboy and miss martian at there school (the one we see alot of)

He was a friend of the leaguers and he ran into mount justice looking for them, but didnt realise the joker was following him and thats how he got in and set a trap for the league

As for artemis, i'm sure she knows her way around the cave enough to find weak points in the security system, even though it is pretty hard im sure, it is possible there is one