1) Population % isn't much of a problem; it should be more like a dozen or so dedicated melee dwarves in most vanilla situations.
2) Like most people have said, three-dwarf squads on constant training schedules will show good increases after a year or two. Try embarking with a pair of trained dwarves in whatever skill you like most (spears, hammers and axes are the mainstay weapons, although you can go crazy with other types if you want), and be sure to keep every dwarf in the squad armed with the same weapon.
3) You'll loose dwarves, no matter what. If you prepare and train, though, your fatality rate should be manageable to the point that new recruits can be trained before the next attack.
4) No. Female dwarves with babies will loose them, yes, but they'll loose them to attacking creatures that aren't trying to kill them; baby shields are effective purely because they absorb attacks. Happiness shouldn't be an issue as long as you memoralize the dead children and keep the mother in a good environment.
5) Steel is actually overkill against most vanilla enemies, and (bear with me here!) full steel armor even more so. Armor will slow your dwarves down immensely, so each of them should have minimal armor (I go with a breastplate and helm). Having good shield and weapon skills will save them much more reliably than armor.
One important thing is to ensure that you set up your defensive positions properly. Your entrance should contain a defensive chamber, as well as positions that marksdwarves can fire from. You should have animals pastured outside to warn you about ambushes, and the most important feature; a way to seal it off, which should probably be a raising drawbridge. The final step is to have large squads of marksdwarves, who can and usually should be untrained rabble drawn from your workforce. When you are attacked, deploy your melee squads to this chamber and get your ranged squads into position. As the enemy files into the chamber, seal off a decent number inside; a dozen well-trained dwarves can easily handle three times their number of goblins, especially with support from your marksdwarves. While an individual marksdwarf will do little, twenty of them will cripple the enemy forces with painful body shots, leaving many of them passed out or otherwise incapacitated and easy pickings for your melee soldiers. After that, let some more enemies in and repeat until they're all dead. This prevents the enemy from swarming your dwarves, which is the best way for them to kill you. You will never have enough military dwarves to outnumber a siege unless you're playing a large and long-lasting fortress, and even larger ambush waves can overwhelm them. The key is to segment their forces and deal with them in a more controlled manner; without the press of numbers they can bring to bear, a goblin is little threat against a dwarven soldier.
Ways to augment their survival include weapon traps (serrated disks) applied liberally to your entrance and guard animals; dogs in particular are good, for the same reason baby shields are. Every enemy that starts killing a guard animal is one that isn't killing an actual dwarf.
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Topic: how large should my military be? (Read 6855 times)