Minecraft(Java edition) Mob farm will not kill spiders

I built a mob farm following this tutorial:

The farm works as intended for killing creepers, zombies and skeletons, but it does not kill spiders.

My best guess is that spiders climb up the walls to avoid getting killed, as seen here.

Is there any way to circumvent this problem?

Thanks!

Solution 1:

I believe you've made an error in the design of your farm. There should not be water flowing into the hole where the campfires are located.

Take a look at 3:34 in the video, there is no water flowing into the hole, just to the edges. This should fix your issue, as even though the spiders will still climb the wall, they will eventually hit the campfires enough to die.

Solution 2:

Did you follow the directions at 7:27?

The subtitles at this timestamp read, "Then go up 100 blocks above the farm".

Take a look at this diagram:

As shown by this diagram, going up 100 blocks above the farm serves 2 purposes:

1. Prevent mobs from spawning underground

Most of the underground caves are outside of the 128 block range, so any mobs in them despawn immediately.

2. Prevent the mobs in the farm from moving

All mobs outside 32 blocks don't pathfind. Spiders only climb on walls when attempting to pathfind somewhere.