The Cortex is a Virtual World from Code Lyoko Evolution created by Professor Tyron.
It is possible to enter the Cortex through the Network using the Skidbladnir.
Unlike in Code Lyoko Evolution, the Cortex's structure does not change.
Once you arrive in the virtual world, you might try going inside the Cortex's core in the center of everything. Although, your access will be denied.
However, throughout your exploration, you may find the Cortex's Tower.
This is the only tower in the Cortex, and it is permanently activated by Tyron. It will automatically reactivate if it is deactivated.
Interacting with the tower's interface will allow you to retrieve data from the tower, which will grant you 20 Energy, but more importantly, you will now be able to access the Cortex's core.
XANA may send monsters near the tower to attack you before you can leave.
After collecting data from the tower, you will find that you are now able to open the door blocking the access to the virtual world's core.
In the core room stands an interface in front of three big screens. Right clicking it will start a data transfer to Lyoko. If you are playing with friends, only the player that collected the data will be able to use the interface.
The transfer will run smoothly until Tyron sends Ninjas to stop the process. They will have to be eliminated to complete it.
Following your battle, the player that started the transfer only has to right click the interface again. Once this is done, the mission will be complete, and you will be able to leave.
Note that completing the Cortex mission does not change anything and is only an optional mission that can grant you an advancement.