My steampunker won't spawn even with all of the requirements fulfilled?
Solution 1:
The house should be at least 10 block wide by 6 blocks tall, it should have any material wall type, example being woodwall, stonewall etc. you need reasonable lighting (at 6x10 1 either end of the house will do), a door, a flat surface example being a piano, table, workbench etc. Once you've met this criteria any cpu can move in. Excluding the Mushroom NPC. I am pretty sure you must kill skeletron prime first though.