A very simple fortress farm that relies on fortress mobs pushing each other into portals. Serves as an upgrade to the Instant Wither Skeleton Farm. This video addresses many common errors and questions. *I believe this farm works for Java 1.16.4+.

RATES (per hr): 128 wither skeleton skulls, 5710 bone, 4180 coal, 1960 blaze rods, 3000 gold nuggets, 83 gold ingots
Pack spawning upgrade (690 additional nether bricks and 624 buttons): https://tinyurl.com/yp9pj773
Upgraded rates (per hr): 148 skull, 6600 bone, 4830 coal, 2230 blaze rods, 3300 gold nuggets, 92 gold ingots

0:00 Intro
0:25 Materials
0:45 Mechanics
4:21 Finding a Build Site
8:42 Managing Wolves
11:43 Build Tutorial
25:25 Instructions for Use
26:20 Multiplayer servers
27:00 Outro

IanXO4 Discord Server:   / discord  
World Download: https://tinyurl.com/657hm2ca
Litematica Files: https://tinyurl.com/2e7x7k3z

REFERENCES
Instant Wither Skeleton Farm.    • Minecraft Elegance: Instant Wither Sk...  
Dark's simple chunk loader.    • The Simplest Portal Chunkloader  

NOTES
0:42 If you are *upgrading the instant wither skeleton farm*, you can reuse most of the existing spawn platform at the same y level. The overworld bridge can probably be reused, and you should add the roof over the bridge to protect blazes from rain. Completely rebuild the player station.
1:40 Some modified servers like PAPER or SPIGOT have a setting to change the hard despawn radius from the default 128 blocks, often to 96 blocks. Ask the admin to change the setting back to 128, or lower the player station by the corresponding amount (e.g., lower by 128-96 = 32 blocks).
3:30 Some modified servers like PAPER or SPIGOT will limit processing of mobs in dimensions that have no players, so this will require a second player to be near the overworld bridge in order for the farm to work. Alternatively, move the killing chamber to the overworld if you wish to commit to a 2-player farm.
4:10 If the storage system fills up, the filtering system will break. Completely empty all hoppers and droppers to fix.
6:13 More details for going up to the nether roof.    • Minecraft Elegance: Instant Wither Sk...  &t=285s
7:00 MiniHUD does not work on servers unless it has Servux installed. Use a copy of your world with the same seed as a workaround.
9:57 The world spawn is where compasses point towards by default. Spawn chunks are the 19x19 chunks that surround the world spawn, which is in the middle.
10:53 Breeding 12 wolves , starting from 2 wolves at full health and 137 meat:
1) Breed wolves 1&2, and feed the pup 13 meat.
2) When the pup grows up (in 5 min), breed wolves 1&2 again. When the pup grows up (in 1 min), breed wolves 3&4, and feed the pup 14 meat.
3) When the pup grows up (in 4 min), breed wolves 1&2 again, and feed the pup 32 meat.
4) When the pup grows up (in 1 min), breed all 4 wolves, and feed the 2 pups 13 meat each.
5) When the pups grow up (in 5 min), breed all 8 wolves to make 4 pups. Consider whether you want extra wolves in case of problems.
12:05 Players load the destination dimension for 15 secs when going through a portal, even if the player teleports away within those 15 secs. As a result, the TNT may finish exploding without the need for a junk item to be thrown into the portal, which is why both junk items are recovered in this example.
12:13 Breaking the obsidian frame will also deactivate the portal.
13:32 If you have my Instant Shulker Farm , there is a bug in Minecraft that can make it behave like a mob switch. Check the description of that video to learn about the fix.
13:42 Invisibility prevents blazes from shooting fireballs that can kill wolves.
22:54 The portal may sometimes generate in a deep cave or in the sky over the ocean. Either way, you still need to follow the same instructions: Break the portal that you arrived in, and work your way to the desired coordinates (by digging a staircase up to the surface, or falling into the ocean) to build a scaffolding tower.
26:44 An alternate route for the second player to reach this AFK location is to use the killing chamber portal, but it can be dangerous depending on whether there are distant overworld players to force mobs on the bridge to despawn. If you take this route, make sure that the other player stands next to the iron door while the second player is getting into place, so that mobs do not spawn in the farm and teleport to the bridge while the second player is getting into place. And make sure that the iron door and gate at the bridge are both closed before starting up the farm.
27:05 Baby zombie piglins can run under the floating slabs and crowd near the turtle egg without despawning. The extra slabs direct the baby zombie piglins to instead run into the portal.
27:18 This spawnproofing eliminates the extremely improbable possibility of a ghast spawning here and flying all the way up to attack the player.
27:26 How to reach the spawning platform.