The Carthage Security System was a program set up by Franz Hopper when he created Lyoko. The program initiated a 3 - minute countdown whenever someone entered Carthage. This was a time limit for which the person had to find a Key hidden somewhere within the Core Zone and stop the countdown, allowing the person access to the Elevator or some other place in the Core Zone.
If the countdown ran out before the Key was tripped, the Core Zone would alter and trap the person within it (which has only happened once, to Aelita in Exploration) until another person enters Carthage and reactivates the countdown again. It first appeared in "Uncharted Territory."
When Aelita and Jeremie recreated Carthage in William Returns, they removed this procedure so they would not be hindered in their fight against X.A.N.A.. However on the Carthage Replika, the Key was featured as a defensive device; it would stop an attack on the Skidbladnir.