[2019] What do trap doors and spikes do?

Spikes - Damage any hostile entity that enters them.
Trap Doors - Drop entities on the first walkable (including spikes) block below.

Also, heads up, version 0.8.23.0 has some fixes for these two props as they weren't working 100% before.

Entities will avoid spikes up to a point (HA) but if they cannot find a better path, they will walk over them. Your settlers won't take damage but baddies will. So, it's best to put spikes at choke points you know the bad guys have to go through. Or you can build a pit with spikes in it and then have trap doors that lead into it. This will drop baddies into it where they will die from being impaled or starve to death if they can't escape but survive the fall. And, yes, you may see a zig-zag movement if you put spikes in a checker pattern.