siege weapon field tactics

[Deus ret.]

I only use them when defending, not attacking - because as you said they are just to slow.

On the contrary, I find siege weaponry to be extremely useful when attacking, especially the Onager class. If you are attacked, usually the enemy rushes or moves towards you rather soon, which leaves you very little time to exploit their abilities due to the horrible hit percentage and friendly fire (I think most of you know how it feels to have an important unit hit straight into the back by a flaming pot [?]).
If you are on the offensive, the enemy tends to occupy the highest availabe ground and wait for you. Well, let him --- shoot him to pieces resp. ashes with your onagers (deploy more than two to really bring them into effect) whose missiles have a greater range than any of their archers'. Keep your machinery protected by a line of spearmen in case the enemy abandons his position.
Not only is this a nice spectacle if you use flaming missiles, but it will likely grant you a much easier victory than otherwise because all you basically have to do is to clean up afterwards, preferably with your cav. Just cease fire soon enough to let your troops get towards the enemy safely.

Sitting out victories .... nice thing.

@cruix,
if you are out for that special kick: okay, machines won't so, but British Head Hurlers. The heads could have been made more distinct, though.