What significant complexity did the Romans add to the pin-tumbler concept to create the warded lock?

Answer

Wards—internal obstructions and projections

The Romans adapted the pin-tumbler concept by adding wards, which were internal obstructions requiring the key to have corresponding notches to pass through unhindered.

What significant complexity did the Romans add to the pin-tumbler concept to create the warded lock?
Originsecuritymechanismlockkey