You have made a good start and I look forward to more. Most of my suggestions are for things you've already addressed in your logs.
If you are set on the hub idea with door entrances to levels, then I suggest a door at the end of each level that only opens after the key is acquired. This door could next to the key or in a different (hard to reach) location for multilayered levels, and then going through the exit would open back to the hub for a smooth transition.
Just an idea, but thanks for the demo!