I played with the shader graph (in URP) to get enemies (and the player) to show behind walls. Then I thoughtlessly added barriers to hide enemies.
Notice the little placeholder “book” in the secret area. Not too secret, but a player playing quickly will miss it.
The pit has a NavMeshLink on it so the player will jump in the pit to the lower level. If the player wants to go back up, he has to go all the way to the stairs on the other side.
This is actually a second attempt. The first attempt I couldn’t get the walls right for a third-person view. I tried making them see through in places, but that only worked from some angles. The second attempt, I decided, make the walls very short, just show that they’re supposed to be walls, sacrificing realism for gameplay.