Minecraft wiki entry for Trapdoor_uses, associated text: 1 Drawbridge
2 Sewer
3 Entrance to Underground
4 Minecart Entrance
5 Anti Zombie Door
6 Retractable Stairs
7 Traps
8 Launching Pad
9 Aesthetics
10 Distributor
11 Minecart Door
12 Entrance
13 Long Distance Wolf Caller
14 Planting Pots
15 Terraforming deserts, Clearing gravel
16 Mob Spawner Trap
17 Spider-Proof Walls