Consider this pit:
During a Goblin Invasion, I noticed that some of the goblins drop down into it. As you can see, it's too deep for them to jump out of, and a little lava and I'd have myself a Blood Moon/Invasion grinder.
However, they don't always jump down. Sometimes, mostly when I'm nearby, they just jump right over the pit. Mostly, goblins coming from the right seem to fall for/in it.
How would I need to build this pit so that the goblins always jump down, other than positioning myself directly under or in it?
Best Answer
Arrogant self answer, because I have done conclusive Zombie science!
Whether Fighters drop down ledges or wooden platforms all boils down to the elevation of the player they're targeting. How near they are in the big picture doesn't seem to play any part in their decision (i.e., X position is ignored).
Three general rules:
Fighters on the same elevation behave similarly to when the player is above; however, against my expectations, if Fighters drop from a higher elevation onto a same-elevation wooden platform, they will pass through. Also, when faced with a decision between dropping through a platform and a jump, they will jump.
So, the reason the above pit let goblins coming from the left through: I was simply on the same elevation as they were, so they jumped it. Coming from the right, they dropped down onto the same-elevation platform and went through.
Armed with this knowledge, I present you a minimalist, player-friendly Fighter AI trap (sans lava and a way of actually collecting loot):
When standing in the middle during a Goblin Invasion, nothing except the Casters can touch you.
Of course, this design can be elevated to allow players some leeway for jumping and getting knocked around by enemy hits. Also, easier loot collection:
More annoying to navigate, though. 4 or 5 blocks high would be a good compromise.
(Disclaimer: Several dozen adorable Goblins, a few Zombies and a Bunny were harmed during the research for this answer.)