A method I use with the Scotts (and other pike-wielding factions) is a pinning formation of Pikes, with Noble (Highland, Swordsmen, pikes, whatever) flankers. The cheap (or not so cheap, depending on what you use) pikemen take the brunt of the force, and their higher defence values let them soak up more damage.
Then a wave of guys with great big swords hits the back of the enemy formation (or the sides, if you arent that patient :P). If done correctly, this generally produces a route against the AI's militia armies. Professionals obviously are less likely to take the bait, and human opponents are harder to encircle since they can concieve of tactics other than the human wave.


In defence of towns I prefer to leave a few units on the walls, then (if the enemy is attempting to go through, rather than over, my walls, stack some schiltroned spearmen on the streets.
Spearmen, even the basic militia units, are suprisingly effective. With three units of militia spears I once routed the better part of an Imperial Army which included a significant cavalry element. Sticking them just inside your gate is ideal, since the AI usually wont charge through a gate, thus removing the AI charge bonus and giving your spears a big advantage.
Another good method is to put a unit just behind a corner on a street leading to the square, this protects them from artillery and archer shots to a degree, and makes them harder to charge.
And, of course, if you find yourself in the midst of the town center with 50 of your previous five hundred defenders left, its probobly best just to order a charge against the nearest enemy than to wait. You'll do more damage that way.

A good overall strategy is to get Timbuktu as soon as possible. Early on load up a ship with a modest army and sail/march to Timbuktu. Dont worry too much about offending the Moors, since youre most likely either already far away from them or (if youre Spain or Portugal) at war with them.
Build a fort on one of those gold mines (or the ivory, I cant remember which is more valuable) and start pumping out Merchants. That extra few thousand florins per turn come in handy.