A waste of time? Oh no, no, no, no, no!

My preferred troop mix for such missions is also "artillery heavy". In fact on occasions so much so that I rotate the artillery units off the field AFTER they have expended their ammo to allow "assault" units on to the field.

I also tend to allow my artillery to keep on pounding away even after establishing the breach - generally either at another tower or at a differnt part of the fortification (say an inner wall, for example).

Positioning artillery is very important (particularly so if using trebuchets or mangonels, which cannot traverse).

I also target towers, for two reasons - 1. it reduces casualties on my side if I take down their artillery, and 2. as Bregil correctly pointed out, you generally take down the adjoining wall anyway.

As for troop types, cavalry still do have their uses here. On occasions it can be handy to have mounted sergeants or the like, who can close to the fortifications much faster than footsoldiers. All depends on what has survived and is waiting behind the remnants of the walls.

Better protected (but still expendable) troops take the van, followed by faster troops which I try to get onto the enemy flank (much the same as an ordinary battle).

The casualty toll will invariably be higher than in a "normal" battle - so I tend to throw "lesser" but still capable troops at the walls while keeping my elite well clear.

I have not encountered the enemy hiding in the forest thing in MTW. Sadly, I have also had relatively few opportunities to defend a castle in MTW - the AI rarely seems inclined to assault my fortifications (must be those scary gargoyles on my outer ramparts!).