PC Monster spawn on player created background wall?

[starseeker]

Steam or GOG

Steam

Single Player/Multiplayer

Multi

Operating System

Windows 10

Terraria Version

1.4.0.5

Controls Used

Keyboard/Mouse

And a picture to show where I stand, which is the green circle, and where the slimes come from, the red arrow.

https://imgur.com/a/AoZCtOd

As you can see, the monsters come from the left, where I have the background tiled off with slabs. The entrance is also closed, yet the slimes keep spawning. Is this a bug or am I missing something?

 

[Geoman265]

If there is a hole in the background wall, even just one block, then enemies may be able to spawn from them very occasionally. From your image, however, I see no sign of holes in the wall.

 

[quixilver87]

Admittedly I do not know this to be 100% factual, but, In my experience, player-placed walls do not 100% deter spawning unless they're part of a 'suitable housing' structure. whether this is a bug or not is not clear.
if you really want to stop the spawning, remove the platforms, or fill that void with lava if you want to even deter spawns from events or solar pillars (which ignore all wall restrictions when spawning)...

I know it's not what you've asked about, but, In my testing, these types of farms have nerfed spawn rates... Better spawn rates can be had by creating a farm with a safe box above the killing/spawning floors.

admittedly, these are substantially more work to make, but, the ones pictured are the best rates I can get for 1:invasions/events (hoik/teleporter box for fastest spawn to kill time & standing spot for pumpkin/frost moon bosses)
2: dungeon or underground (more conventional 'enemies run to the center' type farm with item conveyor for collection)
(it cuts off the full picture, you have to click view full)

 

[starseeker]

If there is a hole in the background wall, even just one block, then enemies may be able to spawn from them very occasionally. From your image, however, I see no sign of holes in the wall.

Yup, I've rebuilt the background wall once to make sure, still keeps happening.

Admittedly I do not know this to be 100% factual, but, In my experience, player-placed walls do not 100% deter spawning unless they're part of a 'suitable housing' structure. whether this is a bug or not is not clear.
if you really want to stop the spawning, remove the platforms, or fill that void with lava if you want to even deter spawns from events or solar pillars (which ignore all wall restrictions when spawning)...

...

Thanks for your input tho. The bottom part is left empty on purpose to fight hardmode mimics, which teleport through walls etc. I've seen the farm designs you posted around, they seem interesting and might build one later, but for me this build has worked perfectly in the past. No issues like this before, so there might be a bug that is causing this, which is why I'm posting in the first place.

 

[Nakano15]

This is really a mysterious case, but try checking the following:

1. See if on the left border of the area, mostly on the thing that looks like a passage way missing switches to the left, doesn't has any monster spawning wall tiles. Maybe the monster is spawning inside the wall? Beside doesn't makes sense, and also you're quite far from the left wall when at the farm at the end of the video.
2. Try increasing the upper border height by one block, maybe the slime is spawning and falling from the blocks?

If you manage to have Binoculars somewhere, you can also investigate the spawn issue by having It selected on your inventory, and looking in the direction the slimes spawns, you may end up finding out what is happening.

 

[quixilver87]

binoculars and the sniperscope/ranged weapon combo both change the spawning mechanics substantially. this in itself may directly deter the spawns he's trying to investigate (while making his farm near useless when those zooming items are equipped).
other than that, yeah, I've seen slimes slip through 1-block gaps and spawn in blocks before, so both of these suggestions are good. The floor of your kill chamber should be 3 thick. door too. Those natural walls could use a background wall aswell as Nakano said.