Sauromatae is my new favorite faction.

[Titus Marcellus Scato]

I find I take too many casualties, even when I just run and shoot instead of standing and fighting. I bring the stack from Gava down so I can field three FMs in combat and let them soak up the casualties so I don't have several units of half-strength horse archers I have no money to reinforce.

If you assault the town, you will take too many casualties from the foot archers on or near the town square - their altitude advantage from being on the hill gives them more range than your archers. But if you besiege the town, and end the turn, the garrison will sally out to attack you - their units will be round the edges of the town and thus more vulnerable. Also your horse archers should move away from the town, and lure the enemy into chasing them. The enemy units can then be isolated from each other and picked off one at a time. Also unarmoured horse archers should be moved quickly out of enemy arrow range whenever they are being shot at, let your FM take the casualties. FM charges against enemy archers (both foot and horse) is also a good way to stop them shooting, they can't fight in melee and shoot at the same time.