Iron is a very important resource in the game, especially if you want to automate things. They are needed for hoppers, minecarts, rails, and many more components I use by the hundreds. It's not incredibly rare, but finding enough by mining can take a long time. Thankfully Iron Golems drop 3-5 ingots when killed, and they can be generated endlessly.
When villagers see a zombie, they get scared and spawn an Iron Golem to protect themselves. This can be facilitated by rooms like this, where the villagers and zombie are separated by wall so the zombie can't attack the villagers, but they can see each other through a small hole:
There are some conditions to spawning a golem though: 1. The villager has slept in the last 20 minutes 2. There are at least two other villagers around 3. The villager has not detected an iron golem in the last 30 seconds 4. A valid spawn point for the golem is found
1 & 2: this is why there need to be three in total and why they need beds. 3: that's where the distance between spawn rooms come into play.
Villagers detect a golem when it's within 16 blocks of them. Using one kill chamber for multiple spawn rooms makes collection of the drops easier, but if a golem spawned by one room moves within the detection range of another one, it resets the cooldown of both, reducing efficiency. To make sure each room can spawn golems at the maximum rate, the rooms and paths towards the kill chamber must not overlap within 16 blocks of any spawn room.
4. A valid spawn point is any solid block within 9 blocks horizontally and 7 blocks vertically. When built on the ground, or in my case on the top floor of the tower, the golems could spawn all around and stay alive within the detection radius, thus disabling spawning new ones. To ensure they only spawn within the water streams that flush them towards and into the kill chamber, the spawn rooms have to be built at least 7 blocks into the air.
The fence gates above the kill chamber stop the water from flowing down and turning the lava into obsidian, while allowing the golems to pass through and drop down into lava, burning them to death. The bottom of this column consists of a 3x3 grid of hoppers to pick up the drops, with a grid of signs on top to create a gap between those and the lava. This is important because lava burns items too and would destroy most, if not all, drops if placed directly on top of the hoppers. But this gap allows golems to drop their loot below the lava and for all of it to be collected.
The hoppers can then either lead into chests or into an item sorter, to separate the iron ingots from the poppies also dropped by the golems, and optionally compost the poppies directly into bonemeal.
