Here are some basic tips, updated for the current gameplay.
1. Mommy is really, really designed for people who have beaten Native, or have won on Normal at least once. Don't start a game on Mommy until you've learned how to fight, both tactically and strategically. Especially don't start a Mommy-level game as a Peasant Revolutionary and expect that to work out. It's deliberately as hard as possible without being completely unwinnable, and it's still buggy.
2. Before trying to become Emperor, you need to amass resources sufficient for a long and difficult campaign. I typically recommend that people amass at least 200K dinars before trying to take any fortresses or towns, and I strongly recommend that you buy Enterprises in every town, so that you will have at least 15K / week of "slush money" that can be used to pay garrison trooops.
3. Promote Companions to Lords. It's practically a requirement, once the game gets serious. Don't be afraid of giving away all of your fiefs- if you actually need the rent from them, you're probably doomed.
4. Gain as few levels as possible. The enemy gets harder as you gain levels. It's tempting to build your character up a lot, but it's a double-edged sword.
5. Fight the enemy in the field. If you are stuck defending a single town vs. an enemy horde, you have already lost, and you should accept the loss and make a strategic retreat.
6. Never take one enemy town or fortress in a campaign. Take a minimum of three, preferably five or more, in a series of lightning strikes.
7. Do not be afraid to build a force that is specifically designed to win sieges but is lousy in the field, and vice-versa.
8. No enemy is unbeatable tactically. What defeats you is poor generalship- poor planning and poor anticipation of the enemy's goals and tactical strengths. An impatient or neglectful general will lose- do not strike until you are properly prepared.
9. Some classes have morale difficulties or bonuses. All leaders are not equal.
10. Troops in garrison cost far less than troops in the field. That said, you really don't want to keep more troops in garrison than is absolutely necessary. The ideal campaign is fought outside castle walls.
11. Sometimes, you are going to lose a fortress or town, due to overwhelming enemy forces. This may happen despite the best possible plans. The best solution is to defeat them in the field; if this cannot be done, attack one of the enemy's towns or fortresses while they are busy defeating the garrison.
12. Enemy Lords who have been completely defeated will not be able to hire new troops until the week ends. You can buy considerable breathing-space by defeating a few Lords in detail, preferably away from the main horde.
13. You will lose whole armies in battles that go awry. This is why you must save enough money to recover and strike again in a week or so.
14. Mercenaries are very powerful and versatile, but faction forces are usually far cheaper.
15. You must keep an eye on morale at all times. You should never be surprised by the state of your troops' morale.
16. Fabian strategy works, but the cost is usually too high.
17. Defeating the enemy in the field but not taking their fiefs is the road to ruin. Their troops are expendable; yours usually aren't. Their fiefs are not expendable; yours usually are.
18. Never start a war you aren't prepared to finish.
19. If playing one of the classes that can take peace offers, be wary of accepting them when the enemy is on the ropes; it's tempting but it usually ends badly.
20. Don't get fixated on territory until you are sure you can hold it.
21. Repeated raids can reduce the enemy's fiefs to a marginal value, at a profit. Massacres are even more effective, but will enflame the enemy to defeat you.
22. You will earn far more than your fiefs will bring in by defeating a small force of Lords in battle.
23. If the enemy takes one of your towns, but loses 3 castles, you have won the greater victory.