[Skarg]

I particularly agree with Lars on 3). That is, Knock and even Open Tunnel do not cause a big adjustment to how hard things are to protect compared to the real world, where burglary can also break into almost any house, and explosive demolition can get through safes and walls, and even an unloaded or fake pistol, or a piece of paper threatening violence, can be used to rob a bank.

Again, security can be provided by guards, a building that's not just one simply-locked door keeping thieves away from the money, and the desire for self-preservation of would-be thieves. If you're smart enough to figure out how to rob a bank, mostly you're smart enough to figure out that you're liable to get your life destroyed if you do.

For example, a bank, even with zero magical protection, might be:

* Located in a safe / wealthy / noble part of a city, which has various vigilant people and a guard force capable of shutting down a group of armed adventurers or brigands intent on chaos of any kind. It might be fairly close to a palace or guild halls, etc, so starting mayhem there may be additionally foolish.

* The bank building may be large and stately, with an entrance with multiple doors leading to a foyer, with multiple doors and halls beyond that, and finding out where the vault is may be itself a challenge.

* There will no doubt be some sort of guards, whose number and locations may be non-trivial to discover, and their first job will be to raise an alarm somehow on intrusion.

* There may be various alarm cords or alarm traps that ring bells that the local authorities will recognize as the "bank under attack" bells.

* The approach to the vault may involve multiple doors.

* The doors for the vault may not be susceptible to Knock anyway. Surely a mechanician can design a vault door that isn't a simple matter to unlock, but involves doing a few things with levers/buttons/etc that a Knock spell wouldn't help with, and/or that may react if someone tries to open them improperly. In fact, it seems to me you could design a door so that a Knock spell will open one lock, but there's a device that responds to that lock being opened, by dropping a bar that no Knock spell will help with, and/or lowering a few portcullises in various places and setting off an alarm, neatly trapping everyone nearby.