An invisible dispenser as this is hidden behind the wall. In the picture above I used the wall open and glass so you can see better how it works. The dispenser can be used well to poison players with throwing potions. But other items are also possible.

Instructions:

Just rebuild the picture but use a different material than glass. If you don't want to throw potions but e.g. blocks, you may have to build more blocks around the dispenser so that the items fly in the right direction.

You can place a few more non-functional pressure plates in the hallway to make it look like you have more traps.