There's no secret key, alas, but it's very easy to force the lock open! And open it does. Behind the III door is a giant clock on the floor. The clock has all of the numbers in place, but no hands.

It seems every number is a switch and two of them are connected to the sliding wall at the back of the room. The others may be connected to other things, and feeling the walls will definitely reveal that there are traps.

Two numbers have to be pressed at the same time for something to happen.

Choose wisely.