Once upon a time I posted a detailed reply to a question about classic iron farming and village mechanics, but a recent search for that turned up nada, so I guess it was one of those lost threads. Since this will be one of my last few posts here before the forum gets archived, I thought I'd offer the details again for those who may seek it. Fortunately I saved the text of my old post.
Since sometime in the early 1.4 series until the recent 1.13 Aquatic Update, village and iron golem spawning mechanics have remained pretty consistent, and back in version 1.8 (Java Edition) I built an "Unbreakable Iron Farm" that supplied a continuous flow of iron through version 1.13.2 with no required repairs.
First, let's cover the basics. The mock villages I used had dual 18x18 spawn floors with a 2x2 drop in the center and 2 villager pods (on opposite sides). This supplied enough valid spawn locations to hit a little over 98% of golem spawn attempts, so I was getting close to the maximum amount of iron, per village, possible. Here is how they worked.
represent air blocks.
are the wall & floor blocks.
are the doors (2 blocks tall).
This is a cross section of the mock village golem spawner I use in my unbreakable iron farm design.
------------------------------------------------------------------------------------- < center of vertical spawn area (+/- 3 blocks)
In this village layout, the center is an imaginary line between the top of the block the door is on and the base of the door. Iron golems will spawn in an area up to 3 blocks above and below that line.
Blue blocks show the 16x16x6 iron golem spawn area within the village.
Looking at my cross section above, you can see that the air blocks above both of my spawn floors are in that valid spawn area, and since they also have opaque blocks below them, iron golems will spawn on both floors.
Every blue block with a full height, non-transparent block top below it is a valid spawn location.
Each mock village had dual villager pods on opposite sides to keep all doors validated, and the pods themselves were carrot farms. This allowed me to move only 2 villagers into each pod, with 1 of them a brown coat "farmer" class villager (farmer, fletcher, fisherman or shepherd), who would feed the other villagers to promote unattended breeding.
Considering how high the upper villages were (top 4 were above the clouds), this saved me a lot of time and effort. The farm pods had an unexpected benefit as well. Over time, and for unknown reasons, villagers seem to disappear from pods, and with my farm pods they just breed a replacement automatically.
Nether portals were built into the pods for moving villagers in later.
Afterward the portal was broken down and the pods were sealed.
More villages = more iron!
Having more villagers in a village may increase the golem cap, but it does not affect the rate at which golems spawn. To get faster golem spawning you need more villages, not more villagers (10 villagers and 21 doors is the minimum for a village to spawn a golem).
Minimum village spacing to prevent merging is 65 blocks, and my farm had them separated by 68 blocks horizontally and 70 vertically. This left enough room for 3 villages to be stacked above sea level, and I built 2 towers of 3 villages, with a pigman spawner in the center for a little gold.
This design ran from version 1.8 through 1.11.1 with only enhancements to the sorting and storage system to increase capacity.
The iron farm I built back in 1.8 still works in 1.13.2 because it didn't exploit game mechanics in any way. It doesn't bend the rules, so it just kept working. It was originally planned to have two towers with 4 villages each, but the lowest villages would need to be built down at bedrock level, so I built the 6 above sea level villages and then procrastinated on the last two, because I was getting more iron than I needed (a lot more).
I have recently built the lower villages underwater and underground near bedrock level. Not because I needed more iron, but simply because it was something I had planned and never completed (now the farm finally feels "finished").
So how do you build a village underwater or underground?
In order for doors to be validated as houses they need more skylight on one side, within 5 blocks. So you need a way to pipe sky exposure down to the doors, and that can be done with air or fully transparent blocks. Light attenuating blocks such as leaves and water do not work. I created 4 light snorkels to bring daylight down to my lower villages (glass columns that reach up to the surface). The design of the mock village is also a little different.
Villager pods in the lower villages wrap around the golem trap area, and most of the villagers are free to roam around the pod.
One villager must be trapped in each corner (behind fence in next picture) to keep doors validated (put good trades in corners).
Only 4 light snorkels are needed for 40 doors below (10 doors per corner, 1 skylight source for each). The trick is to place the doors sideways, facing the corner, so the "front" of the doors are within 5 blocks of the light. The doors were also placed on the outside of the pod so light wouldn't be blocked by villages above (doors are outside the shadow of the upper villages).
The light snorkel is in the corner just above my crosshair (behind the corner doors).
The lower kill chamber and item elevator is 1 block above bedrock level, deep in the bowels of the Minecraft world!
Kill chamber on right, ladder up to villger pod near center, and item elevator to the left (access by nether portal).
Minecraft Java Edition Aquatic Update version 1.13.2 will be the end of the line for the "Unbreakable" Iron Farm, which produced an uninterrupted flow of iron ingots and poppy/rose flowers from version 1.8 through 1.13.2 with no required maintenance or repairs.
Due to the massive changes to villagers and village mechanics in 1.14, older iron farms no longer work. I have an idea how to fix it, but it's kind of a kludge to save most of the structure of the old farm, and not really the way a 1.14 iron farm should be built.
Final 1.13.2 configuration...
with lower villages, light snorkels, and item elevators to bring iron up to the sorting and storage system.
But wait a minute, Courageous Marinade...
Now those of you who've studied older village and iron farm mechanics are probably saying that the minimum distance is actually 64 blocks, and wondering why I use 18x18 floors for a 16x16 spawn area?
Minecraft derives its block location from the north-west corner of a block (not the center), so depending on what quadrant of the map you build in, the village center can shift up to 1 block to the north and/or west. That extra block of separation ensures that your villages can never merge, even if you build across a boundary that shifts your village centers, and the 18x18 spawn floors keep all valid spawn locations inside the golem trap (also, placing the water is easier).
So that's my take on classic iron farming, and my "unbreakable" design, which was not quite unbreakable after all, but it had a good long run from back in 2014 when it was created until a few weeks ago. I hope this info comes in handy for others playing older legacy versions of Minecraft.