Any chance you'd take a look at the economics side of the game next? Current state (imo):
Prisoner farming: By far the best way to pull in cash, all game.
Tournaments: A solid supplement as long as you get max odds on your bets.
Industry: A decent, reliable supplement for a reasonable investment cost.
Fiefs: A couple of villages is a nice supplement, but hardly decisive compared to tournaments and prisoners. Other than for knights and a place to store troops, castles and towns aren't really worth the cost of garrisoning or the hassle of re-taking or beating of sieges.
Loot: Reasonable in the early game, irrelevant by mid game.
Quests: Most cost you pay next to nothing. A few pay enough to be relevant in the first hour or so of a game. Only ever really done for relations afterward.
Early game feels good. Fighting for loot and prisoners, participating in tournaments and doing the odd quest feels pretty decent.
Mid game feels right. You've got your first castle and your industries up and running. Clubbing adventure companies and winning tournaments to build up cash and renown to further your ambitions makes sense. Being able to make meaningful investments in your fiefs would make a lot of sense at this point, rather than stockpiling cash.
Late game, not so much. You've built up a half dozen fiefs but they don't really pay well. Garrisoning castles, protecting villages, and beating off sieges takes a lot of time and men. It feels right, but still needing to farm prisoners and fighting in tournaments to pay your men doesn't. Upgrading your fiefs is a huge cash sink and takes ages, with very little reward.
It'd be really nice if, by late game, the economic game transferred to fief management. Building upgrades, beating off raiders and sieges, hiring sheriffs to patrol for bandits and clearing bandit lairs, sending and protecting merchant caravans, etc. instead of more prisoner farming. You're a powerful lord or a king after all, you should be funding tournaments not fighting in them in the hopes you can pay your men for a couple more weeks with the winnings. And, like every other lord in the game, you should be able to afford reasonable garrisons and field a reasonable army with proper stewardship of your lands.