Medieval 2: Total War:
Huge, angry army stomping your way? No problem, just place a single squad of light cavalry in its path. When the enemy engages, retreat to a corner, then kite the army round in a circle until time runs out. Congratulations, you just beat 2000 heavily armed soldiers with six farmboys on horses.
Also, once you get long range artillery, getting besieged is no problem, just sortie out, roll your artillery just outside the city gates and leisurely blast the enemy to pieces while they stand around muttering.
Edit: Crusader Kings 2: the old ladies strategy and the old men strategy:
The old ladies: Once your ruler gets eligible heirs, get rid of the wife and find a new one. Now, whenever the ruler gets married in CK2, your vassals give either a pile of money or prestige as gifts. So, just scour Europe for the most eligible 80+ bachelorettes and widows. With luck, you could pull off half a dozen weddings in a year.
The old men: Vassals are such petty creatures, aren't they? They get pleased when they get titles, but that wears off in a decade, and then they croak and their heir shows no gratitude
at all. But what's this? If a vassal dies with no heirs, the ruler inherits him? Well, just invite every lowborn geezer 60+ to your court, marry them off to women in their late 40s, and whenever you get a surplus title, parcel it out to the oldest whoreson still around in your court. Within the week,
old men will be running the world!