Be prepared to be hit, and hit HARD.
My current 5 year old FD fort has an uncountable amount of dead siegers on the list.(35 pages) It also has 710 body parts, 592 corpses, and countless bones. More Grimly, it also has 124 filled coffins. My population is 109, with 50 full time soldiers. Pratically all of them have had some form of injury, with the legendaries having many scars. I'd say at least 25 military dwarves have died in service.
There is a dedicated hospital with 10+ doctors,god knows how much of the military they have treated.
My fortress is not astheically pleasing; It is almost entirely geared up for military needs, and there are so many dropped weapons, corpses and body parts they cannot all be collected in time before they rot.
Here are some pics of my layout: (click to enlarge)
East entrance, with 3 drawbridges for crowd control. On the bottom right you will see some weapon traps. You will need good soldiers and good traps to defeat the huge sieges you will face
refuse stockpile, barrack and hospital, the barracks is 4 floors high. I hate doing it, but i have just started using a qauntum stockpile for refuse, far too many body parts..
and finally..
The ossuary. No much respect for dwarves here, no invidual tombs, and certainly not enought room/time for them
You will be sieged heavily, and often. in eyars 3/4 i had 100+ enemies in one siege, a whole mixtures of blendacs, tigermen, strangalers etc.
The tiger men will be the most persistent, Armed with copper/silver and sometimes bronze, they will siege you in huge numbers from year 2+
Blendacs are one of the more dangerous, they are undead, and feel no pain/fear/nausea. They come with bronze by default, the siege this year all came riding GIANT badgers, that sure was intense.
Goes without saying you'll need some way of getting steel.
oh and have fun, this mod brings fact the fun and !!fun!! when normal vanilla enemies bore you.