Arrow keys to move. Use the switch to remove the gray barrier, and collect the key to unlock the door.
The switch object sends a Message to the gray block when the player touches it, using a Collision trigger. The gray block's Mailbox is connected to a Destroyer, so it is removed from the level when it receives the switch's message.
The door object works similarly - the key sends it a message when touched. The door object plays an Animation when it receives a message though, and uses a switch to enable the player to progress to the next level.