I've never played beyond a hundred dwarves so I don't know if things get nastier.
It seems to me that in most normal worlds with sane environments (trees and no alligators), you start getting ambushes around the time of the second outpost liason caravan. So you have two years to go nuts before you need to be ready to handle trouble.
I start cutting wood immediately and go like crazy until the elves show up and cap me. The woodcutter has hauling turned off and does nothing but cut for over a year.
If I am on a flat map, and most maps seem to be, I make a rectangular fort over the main entrance, and have this fort be large enough to hold the main initial wood stockpile. That's pretty big. I build this out of rock rather than blocks, and to deal with construction time I use profile to allow only the mason access to the mason's workshop, but make everyone else a mason, so they'll all run out and build anything in a few moments. The fort has archer platforms on the corners but is otherwise just a normal wall. There's one drawbridge in, then a long fenced path that parallels a fortress wall. This path is loaded with weapon traps. If you get past this and go underground you end up in the barracks, which I figure should give pause to most kobold thieves.
As long as I have this stockade done I can bunker down and not go outside to cut during the 3rd year, which is when things seem to be toughest.
So far this is proof against everything except one goblin wrestler (?) who was an expert crossbowman. He was head-shotting my archers from six squares away while they were behind fortifications.
Caverns I break into and if I'm feeling like I can, I wall off large areas and use this as a safe tree farm. After I do this I move the wood stockpile underground. If I'm not actively messing around in the caverns I wall them in. As far as I know nothing can break through a constructed wall.
Eventually FPS kills me though.