From what I can tell, weapons, armor, and traps are made at the workshop by the gnomes. And does anyone have some tips to stay alive?
Not just gnomes. Any humanoids, as far as I can tell, will go forge or brew things from time to time.
For survival tips, I'd rank up one of your skills a few times, then get a few minions from there. Maybe massing gnomes is effective since they craft their own gear and beds are cheap, but I suspect it's easier to just pay the initial mana to get the good stuff. Mind you, I've been playing on default, which is I guess Easy.
If you're really having a hard time, remember that while the AI is pretty decent about upgrading their equipment, they're not perfect. You can manually possess your critters and order them to pick up and equip stuff, and unequip and drop their other garbage, if they still insist on using those +0 Acc, +5 damage knives while you've got a +5 Acc, +8 Dmg elven sword sitting right there.
On a related note, your imps will only harvest items dropped in your dungeon. If you kill enemies outside, their corpses and possessions will remain outside your grasp until you manually fetch them. Also note that corpses are too heavy for most creatures, so if you're hoping to spawn vampires you may need to dump all of a critter's stuff.
Finally, remember that guard posts are a bit of a double-edged sword. They're very handy for low-level attacks, but they'll distract things, especially golems, from training.
At some point i was wondering why my vampire was missing so much and taking damage, i had equipped him with a very good elven sword (taken from my stockpiles on which the imps dropped the dead invaders items too).
I looked at his equipement and noticed he had nothing equiped, despite previosuly i had equiped the sword he picked up. And then i noticed on the top of the screen :
Makes sense, and reading again the top screen log during the battles i saw that combat is really brutal in a Dwarf Fortress ways, limbs are flying around, and that's then what happened, one of the invader just cut my vampire hands.
This is amazing. I'd noticed a lot of limbs being hacked off and lying on the ground, but I didn't realize they had a practical effect like this.
I didn't noticed having lots of library tiles improving the mana gathering, it may probably allow some summons that may or may not (as i have not tested) require more library tiles, but for mana there does not seem to be any difference. Your haste trick sounds very usefull, thanks
More library tiles are requires for more spell/skill rankups. If you've only got a few tiles, it'll give you the same "You need a bigger library!" message when you try to improve one of your skills as you do when you try to summon more humanoids than you have beds, corpses than you have graves, and so on.