Today I made a fort that uses an askari-based army.
My fort is an above-ground city out in the desert. After my population was twenty-something, the fort's citizenry became a closed-shop and any new migrants were now colonial askari. When a migrant wave arrives at the fort, they are all drafted, given an iron spear, and told to start sparring at the barracks. No armour allowed except for the citizen officers, who wear plate armour and are marksdwarves, and serve to keep the askaris in line.
Threats are dealt with by dwarven waves of spear-wielding unarmoured soldiers. I haven't had any sieges yet, just a few ambushes, so I'm not sure how bad casualties are going to be yet. So far my only military losses have been in sparring (no wooden weapons allowed.. learn to use your combat weapon) except for a couple askari who have been out on patrol and discovered ambushes without backup.
The first askari wave was a pretty awesome scene though. Ten migrants arrive in the desert colony and immediately are drafted and told to go get a spear. Before they can do so, a giant eagle swoops in and kills one of them instantly. Moments later, they see a flurry of crossbow bolts and the eagle drops. The Colonel has just welcomed them to Africa.
I've started rewarding particularly brave and effective askaris by letting them wear leather armour, more for bragging rights than for actual protection. The latest leathered askari won his for getting a hat trick of three sparring dwarfkills. I don't imagine anyone likes doing spear drill with him anymore. I can't wait to unleash him in a siege situation.