Simple Scaffolding mob farm

ianxofour's instant mob farm, built to go up fast and expand as you go rather than to top a rate chart. Its trick is scaffolding: it carries a redstone signal straight up the tower, so one clock on the ground fires the water dispenser on every floor at once, and mobs originally spawned on the scaffolding itself. No perimeter, no digging, and it scales to 25 platforms. From 1.19.3 mobs stopped spawning on scaffolding, so the platforms become top slabs while the signal tower stays exactly as it is.

• Build the spawning platforms from top slabs, not scaffolding, on anything from 1.19.3. The central signal tower of observers, dispensers and scaffolding is unchanged, and builders confirm this working through 1.21.5.
• With slab floors the four trapdoors on each floor stop being necessary.
• The lava column trick at the end can burn the farm down. Lava will not ignite scaffolding directly, but it can set the ground alight and the fire then spreads to the scaffolding, so clear flammables or turn fire spread off.
• Bone meal the kelp rather than placing it by hand, and use falling sand instead of a piston to push the blocks over.
• Check the biome across the whole build area. Beach biomes cause problems, which ianxofour added to the description after a builder raised it.
• Run the scaffolding tower up from the trapdoor rather than the solid block, and build the first platform before the clock so any missed scaffolding is visible.