The Iron and Golden King

Three things in one barony: 60+ income, a stationed Heavy Cavalry regiment of 500+, and a mine. Two trigger details matter: the income check is the barony's TAX value in the holding view (monthly-income modifiers like Greedy don't count), and mines are special buildings only certain baronies can have - the wiki's best picks include Bambuk and Bure in Mali and Göttingen in Germany. Make a mine county your capital and stack holding tax: income buildings, duchy Tax Offices, development, tax-collection activities, Avaricious (+15%), Absolute Control (+10%), and serving as your liege's steward (+10%). Grow the regiment past 500 with size-boosting traditions and buildings. Realistic from High Medieval tech onward; comfortable by Late Medieval - a patience build, not a puzzle.