Keys and doors? If you're using a variable or items for the keys themselves (just like the generic "keys" of Zelda) then you would just have an event for the door.

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Page 1 - Closed Door
No preconditions
Trigger Condition: Touched By Hero
Event Layer: Same as Hero

Pseudocode:

<> Conditional branch: Keys > 0
___<>Take away 1 key
___<> Set switch "xxxx:Open Door 001" ON
___<> Play some sort of sound effect
___<>
:End
<>

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Page 2 - Open Door
Precondition: "xxxx:Open Door 001" is ON
Trigger Condition: Touched By Hero
Event Layer: Below Hero

Pseudocode:

<> Teleport to next area
<>