The Fat Cat achievement is obtained once you’ve earned at least 1 million denars in trade profit. This means that wealth earned through plundering and looting won’t count, but manipulating the Bannerlord economy will. The economy for each campaign will be slightly different, but prices will remain similar at the beginning of a game. What you do from this point will determine which goods become more rare or more common, and how you sell them for a profit will be based on your knowledge of the local economy in your campaign. For a quick guide on how trading in the Bannerlord economy works, see our page on Trading For a Profit.