I've done pretty well with Fortress Defense II (which I have on all the time) using pure military (no traps and no walling in), my setup is as follows (when I say 'squad' I mean a group that works together, I keep my actual squad numbers low so they train better due to training bugs):
Speaking generally, once I have a fort up an running I can usually run it well with around twenty dwarves doing production (food, furniture/good production, weapon/armor production, resource gathering, etc). The rest are all drafted into the military and get sorted according to their skills.
One squad of bladed dwarves (axe, sword, etc),
One squad of blunt dwarves (hammers, mace, etc),
One squad of piercing dwarves (spears, etc),
Three squads of crossbow dwarves,
Breed lots and lots of war dogs,
The rest of my non-production essential dwarves get drafted as cannon fodder infantry, they get set to use any melee weapon they want.
I set up walls so there is one opening 3x3 wide at one side, I build platforms on either side of the opening and keep my war dogs pastured in the opening itself to catch people sneaking in (I keep a few inside for breeding).
When a siege comes I get all my production dwarves to take cover inside and send the military out. I put one squad of crossbowmen on one platform, the other on the other platform. I place my fodder out with the war dogs to kelp soak up some early kills, then I have my primary melee dwarves behind them. Behind my melee is my last set of crossbowmen.
The enemy comes in and starts getting peppered by bolts as they get close and come through the gate, then they run into dogs/fodder and hopefully turn their attention to them, then my main force attacks full on (hopefully to distracted enemies), and my archers behind help finish them off.
This has been pretty effective for me and adds the extra challenge of not using walling-in or traps.
For general equipment obviously 'full steel/adamant' is best but failing that you should make shields and helmets your primary armor to make, once everybody (or at least your main melee guys, I sometimes outfit my archers but seldom truly 'outfit' my infantry. If they happen to find a loose helmet from an enemy or unfortunate primary melee dwarf they can use it but I don't go out of my way, their job is to soak up damage and distract arrows) has those start on everything else as shields do really well to block/attack and head-shots are quick-kills.
For industry I make sure I bank a lot of wood so I can get my armor/weapon industry up and running quickly and if I get sieged in I have some to work with, I also get a good butchering industry going so I have plenty of food stored up and bolts as well. I make sure I have an interior source of water and a hospital with plenty of supplies to treat my wounded (its worth it to save a really skilled dwarf). I try to start my military ASAP even if its just two dwarves training and often try to start a few dwarves with some skill in various weapons to help train others (I don't know if that actually helps but it couldn't hurt). Lastly you can gather up useless equipment from enemies and/or make goods to sell and get some extra metal and/or weapons/armor from the caravans. Also sometimes I'll 'modify' my no walling in rule and allow floodgates or doors to block my entrances. These are still prone to building destroyers so its not 'true' walling in, but can help block weaker enemies giving me more time to set up my military for defense.