SCP: Breached Containment is based on Site-19, during the construction of Site-19, it was under supervision by Dr. Maynard a spy that purposefully built the site to be dangerous so the Chaos Insurgency can invade it more easily.
After the previous containment breach they programmed C.A.S.S.I.E. to detect any intruders on the site during future containment breaches and Dr. Maynard was not heard from again...