I had a lot of war this week-end in Civ 4 and it's as many of you said: the most basic strategy is to send the siege units first to inflict collateral damage, then clean up with soldiers.
It's what the AI did to destroy my stacks a few times, and I finally understood why it made so little sense to me. First, it seems like the AI would rather suicide its siege units than bombarding with them (ranged collateral damage being possible with RevDCM); then, with a unit limit per tile, the scale of the fights, although more realistic (to me) doesn't match the fact that a single tile can be attacked a infinite number of times per turn. In Civ5 this isn't much of a problem because a unit can live through a fight and still occupy a tile; in CTP the battles are stacks vs stacks anyway.
That's why the concept of a frontline, more enjoyable than the stacks of doom (to me again) gets nullified because the enemy can just annihilate a tile even though you have units all around. In ROM AND, splitting your forces can give you a "surround and destroy bonus" but it only works on the offensive side; I think that defense could use some similar bonus when your opponents blindly attacks the middle of your frontline.
Anyway, some other thing bugs me: when you attack a little stack and there's only one unit remaining, what do you do? Sometimes I feel like if I clean it up with my last available units they will get destroyed the turn after because they'll be isolated. Sometimes I'm tempted to ignore it because he can't take on my own units, but I'm getting pillaged if I don't do anything.