Actually I forgot one thing which is well worth mentioning. Embark with two proficient weapon users. Put them in a squad together. Don't give any time off. Initially give them a weapon and shield, but later give them top-quality gear. By 1 year they will be legendary warriors and will absolutely demolish ambushes. If you kit them out with quality gear like steel/candy, they'll demolish sieges.
Playing without embarking with a couple of mililitary dwarves is like hard mode, playing with them is like easy mode. They are far far cheaper and less work to setup than trap corridors, and if anything slips past their barracks (which should be sited to intercept intruders) they can just be stationed to intercept the threat - unlike every other kind of defense which is static.
In vanilla DF just two dwarves who have been training since the start of the game will turn all sorts of threats into a joke. Even if you're planning on having an impregnable fortress, playing without 2 guys in the military is like playing hard mode, because it gives you no insurance against failure of the static defenses, if enemies slip through they run amok wiping out civvies until some lucky miner manages to dig out their brains - you don't want this. So have at least a tiny elite military to intercept those threats.
I play Fortress Defense mod, and sometimes I have (accidentally) ignored early sieges like stranglers, then I remember the siege, and it's already gone because the military, who I have train on the surface near the entrance, wiped it out automatically. It's like a long-range weapon trap that attacks every nearby enemy sufficient times to kill it. Building big complicated architecture defenses is soooo much more work.