I have only played two campaigns so far, but here is what I do for what its worth.

1. My usual defensive set up for Romans pre Marian against Barbarians, Spaniards and Carthaginians is:

---Slingers---------------------------------------------------------------------------------------------
-----------------------Hastati--Hastati--Hastati--Hastati--Hastati--------------------------
--------Spearmen---------Velites----Archers----Velites-----------Spearmen------------
-----------------------*Hastati*-Hastati-General-Hastati-*Hastati*----------Wardogs--
---Equites----------------------------------Equites--------------------------------Equites-------

Slinger to pelt them as they approach, wardogs to attack any missile units that the AI sends up front (though I do feel a little exploity using them except against rebels), mercenary spearmen to beat of any flanking cavalry. I let the enemy come up and bash the hastati before flanking round with the *hastati* (the two most experienced units) and equites. Genrally also I have the hastati in the second line and the missile troops focus all their fire on one unit at a time. Usually I can get three enemy units to rout bfore serious fighting. If the enemy general charges at any point, I will give him one volley from the velites and counter charge with my general. Otherwise too many hastati get killed.

If fighting against chariots I will go in with a couple more velites and put them on the flanks to swamp the chariots or hide in woods to emerge and attack from the back.

2. On the attack, it really depend on the terrain.
- Never attack up hill or head on. Maneouvre to attack along the top of a ridge.
- Wait to let your troops get back to fresh or atleast warmed up before final attack.
- Lead with velites or some other missile unit in loose formation to absorb missiles - casualties matter less as there wont be much time to bombard the enemy with missiles anyway and velites are cheap.
- Always attack one flank first with infantry and with cavalry sweeping around, archers/ slinger bombarding the rest of their line (velites dont have enough range). As the AI redeploys to line up face to face with you, look for opportunities ot pick off units, charge missile units with cavalry or infantry units which can be attacked quickly from all sides and routed.

3. When sallying out in a siege, lead with cavalry first. As the AI redeploys (why doesnt it shoot as you emerge from the gates?!!?) charge whatever unit is at the AI's tail. With 3 units of cavalry it will rout and at most the penultimate tail unit will move back to support it. Run the cavalry off a little distance and then charge and rout them too. By then your infantry will be outside the gates and the cavalry "warmed up" so give the horsies a rest and use the infantry to start the main battle.

4. Siege assaults: anyone have good strategy for this? controlling units i find is a nightmare they keep going the worng places, find the worst routes and engaing units they should be skirting around. Ugh.