While I cannot take credit for the incredible anti-intrusion defence system described below, I can attest to its effectiveness, based on the article I was reading at the time I came across the description of said system.
I heard about this guy who broke into a lion’s den at the zoo and got mauled. People were talking about how there should have been better defences put up to prevent people getting into the cage. A friend of mine suggested putting some sort of fierce animal in the cage, which would attack anybody who climbed in.
Unknown Comment Writer, c. Nov 2011
[Photo Credit: AbZahri AbAzizis from Kuala Lumpur, Malaysia (2010-01-30 – Outing – ZooUploaded by Snowmanradio) [CC BY 2.0 (https://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons]