I have just read pretty much all of the posts here and I have certainly seen some interesting things, but i have also found that there are a lot of easy strategies missing. since i don't have a lot of time at the moment, ill name just a few for starters.
first off the fact that the formations, the units, that is, do not disperse. hereby i mean that in no situation do they move out as invidual characters, and stay together all along. this brings some adventages when defending.
if defending a breached gate, or a breached wall ofcourse, the enemy units are forced through a space which can be held by one or two units of yourself. to kill of a huge number of oncoming units, you can do the following;
place a single or when facing great numbers two heavy infantry units (preferrably something with defence 21 or higher) to hold them at the spot, just 2 normal units withs from the breach. by doing this, they are inable to go past those 2 units, just because of the fact that they have to hold together.
because of the space between the breach and the defending unit/units, you can kill them all doing the following; place 2-4 groups of archers (in case of england or france, 2 will do) at both sides of the defending units, at probably a units length, so they can fire into the flanks of the already engaged enemy.
and trust me, this works, even against cavalry.
second, i have (again a defender's) note for defending castles up to citadels in rocky or mountainous area's. in most cases, u'll find those castles build upon a hill (duhh, height works your way). ofcourse it helps that your archers can fire further, but the point is that the roads in the castle itself exist of horizontal pieces and those that go up a slope.
since i specialise in defending (well, open field battles aren't that hard, swarm than cav, always works, and i rarely assault an enemy's castle) I thought out some idea's for holding the enemy out, but also some for letting them bleed as hard as they can. this is one of the latter. you can place several groups of units (1-2 spearmen(but ofcource preferrably something stronger), 1-2 archers) to defend the roads they will most likely take to go up to the square. here, you let the spearmen collide with the enemy, and the archers rain hell down upon them from above, this way you can kill many, without using your best troops.
well... this was it, ill be posting more in a while, hope you find it useful