A Control Tower is a small tower in a cities port capable of opening the system to the Net. It does this by firing a massive energy beam at the cities sky, creating an opening to the Net. Any system with ports to the Net require a Control Tower to open itself to the Net, without it Net travel to and from that system is impossible.
Mainframe's Port and Control Tower are defended by heavy anti-aircraft guns and a squadron of CPU fighters. The Tower itself consists mainly of a control room at the base and the tower is filed with equipment and emitters to generate the energy beam.
When Daemon began her complete take over of the Net she had the Guardians enter systems and destroy their Control Towers so that they could not seal themselves from the Net. This way she could freely enter any system she needed to. (Cross Nodes)
After Mainframe was opened to the Net and its Control Tower was destroyed Mouse resealed the system by activating a firewall around the hole in the systems sky. By encrypting it properly no one could gain access to the system. (Daemon Rising)
There are a few other ways to enter a system that is closed to the Net, but they are generally unreliable. Portals can enter a system, but are very difficult to create. The Gateway Command is capable of entering a system like portals do, but there is only one functioning Gateway in existence. Game Cubes can enter any system with or without Net access, however there is no way for a person using them for transportation to control which system the Cube enters next. It is random chance for a Cube to enter any particular system.
- The Control Tower is modeled after the BT Tower (British Telecom) located at 81 Newgate Street, London.