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The key in Carthage is part of the Carthage Security System.

Core Zone

Carthage had a security system prior to Season 4. When someone went into Carthage, they had to press the key in time to access the rest of Carthage. This key is located somewhere in the Core Zone. The User only has 1 - 4 minutes to trip the key. If the user doesn't trip it in time or leave in time, the Core Zone traps them. This has only happened once to Aelita in Exploration. The Key was not known until Exploration.

Chamber Key

After a person enter the south pole of the sector, they have to trip a key that unveils the stairs to the chamber. Unlike keys in the Core Zone, this key never changes its place and it isn't hard at all to reach. Stairs might have time limit since key must always be activated to make them appear again. This is the only one that still exists.

Post Lyoko's Destruction

In William Returns, it was announced that Jeremy and Aelita removed the Core Zone Key when they were rebuilding Lyoko so it wouldn't get in their way fighting X.A.N.A..

Carthage Replika

Like in Sector 5, before Lyoko's destruction, Sector 5 Replika (with possibility of many more of them) has a key somewhere inside it. This key is not only connected to rooms, it also connects to X.A.N.A.'s firewall trap used on the Skid. The only key appeared in A Space Oddity in which the Lyoko Warriors discovered and destroyed the replika. The key was pushed by Ulrich who was fast enough to find it.