There are many, many threads across the web about how impossible this achievement is in The Sims Medieval... Lately I had a whim to play this game again and I just managed to get it, so I'm returning the favor.

As to not awaken decade old threads I'll write it here for fellow nostalgic simmers to find:

DO NOT. BUILD. ANY. BUILDINGS!! EXCEPT EITHER THE BARRACKS OR THE SPY QUARTERS.

Quest as you please with either the monarch and the spy/knight, once you've reached 500 Renown you'll unlock the Fistful of Heroes achievement.

That's it. Happy simming.