My first ever "siege" in v40.xx came in the form of a group of bandits. Since half were packing ranged weapons I resigned the fort to its demise and assembled an adhoc militia of 5 of the bravest dwarves out of a population of 17 (or 20, I forget which, but it wasn't above 20 population, one of which was a miner and the other a woodcutter.) Armed with a sword, mace, and warhammer salvaged from the ruins of the reclaimed site and a pair of work tools, and virtually no training (save the swordsdwarf who had arrived with some and a few goblin kills from worldgen,) I sent them out to face the enemy, who had them outnumbered by 4; though granted 3 enemies were wrestlers and two were recruits, I expected the enemy bow and crossbowmen to clean house with my tiny force.
And then it happened.
A wrestler rounded the corner of the fort's overland building, and meets the untrained (but strong thanks to a full year of working with fairly solid woods as a carpenter,) macedwarf. Who delivered an immediately fatal punch to the brigand's chest, as the blow caved it in. The woodcutter dropped to his knees, overwhelmed by the sheer horror of what he just saw the macedwarf do. Two more wrestlers rounded the corner and one was set upon by the macedwarf while the other began fleeing in terror.
A mace to the head ends to second quickly, and the rest of the militia catches up, save Woodcutter McUnderstandablyHorrified. The hammerdwarf gives chase to the fleeing wrestler while everyone else charges the other bandits. Two rangers and a recruit spin right around and run away while two others fire at the dwarves, missing thier shots badly and one getting mauled by the miner, who then collapses under the horror of what she did. The sword and maced dwarves kill the few brave wrestlers and run down the one enemy melee soldier with a weapon, who turned and ran after the wrestlers died; I think he was lagging behind due to that heavy metal shield of his. From there is turned into a benny hill style chase that lasted the better part of an in-game week before all the panicked invaders were killed and tossed into the town garbage dump.
I had expected my dwarves to be gunned down and punched to death by wrestlers and marksmen, only to have them crush the bandits handily, even if there was so psychological trauma for the woodcutter and miner.
On a separate note, in a different freshly founded fort I had a small pack of children running around when a mandril raid came. The mandrills killed one child. The children killed all the mandrills (A second group showed up shortly after that I sent the militia after, so the mother of the deceased child could have some form of retribution.)