The summary.
SECURITY IS LAYERED
Perimeter, building, zone, hall, rack. Each must be defeated in turn, and each provides a chance to detect.
In a shared facility, other customers' staff are in the same hall — lock your own cabinet.
RECORD EXITS AS WELL AS ENTRIES
So it is known who is inside, which matters in an emergency.
FOLLOWING SOMEONE THROUGH A DOOR IS THE SIMPLEST WAY PAST ACCESS CONTROL
Which is what mantraps prevent.
RECORDING WITHOUT MONITORING DETECTS NOTHING AT THE TIME
It provides evidence afterwards. Access control and staffing are what prevent.
CONTROL WHAT LEAVES, NOT ONLY WHAT ENTERS
Equipment removal is how theft occurs, and it looks entirely normal.
PHYSICAL ACCESS TO AN UNPROTECTED SERVER IS COMPLETE ACCESS
Protect boot configuration and console access, and encrypt drives so a removed one discloses nothing.
REVIEW THE ACCESS LIST REGULARLY
You will find people who no longer need access, and sometimes people who have left.
