Creature banks.

You seem to be familiar with scripts, so I'll just provide a general list of actions:
- Create script tables with guards and rewards (organize them in a way that is convenient for you)
- At the start of the map, iterate through all buildings of this type using GetObjectNamesByType("BUILDING_BLOOD_TEMPLE") and attach touch triggers
- Alternatively, you can create a class for the building-bank with corresponding initializers...
- Touching the bank emulates a battle using StartCombat, and in the callback upon the hero's victory, a random reward is given (or a reward tied to the strength of the guard)
- In the callback, the touch trigger is also reset (or a message is displayed, such as "The treasure has already been looted" + MarkObjectAsVisited()).