My fortress is around five years old with 80 dwarves. The goblins hadn't been ambushing at all, which I though odd, but luckily I had plenty of time to prepare, because suddenly they began an all out siege. And while the first seige was only one squad, now every seige is at least three mounted squads accompanied by trolls. Fortunately my first squad of ten and backup squad of five have been able to wipe them all out easily, with only a few yellow wounds, and one death (when a stupid newbie ran all the way outside chasing an Elk bird, only to run into a fresh squad). Which is lucky, because last year they seiged TWICE during winter alone. My biggest issue has been dealing with all the crap they drop - I barely have time to clean up the corpses, let alone everything else.
Meanwhile, in the non-seige times, I've been exploring the caverns very carefully, as there are SIX forgotten beasts out there. So I've been digging exploration tunnels into the edge of the cavern walls (a few z-levels up, as none of them can fly) but whoops! I accidentally dig into a hidden cavern at floor level, and before my builders can wall it up, one of the buggers snuck in. Fortunately, it wasn't one of the unkillable ones, and my military wipes it out without injury. I send them home, but once again before the gap could be walled up and before the squad made it anywhere near barracks, TWO MORE sneak through, right after each other. I was hoping they might start an epic beast battle, but unfortunately the two were in cahoots, as one headed down the long 100zlevel stair to my fortress underground, and the other heads up 50z level to my old starter fortress, chasing a furnacer (as I call my smelting/wood burning folk). So I send the squad back up to deal with them. The first one (a sauropod) dies quickly, luckily before it made it down to my main fortress, but the other chases the furnacer right back to the surface to my old starter fort and begins whaling into to furnacer. The furnacer holds his own, getting pretty beat up and losing a leg, but still manages to inflict a few yellow wounds on the beast (which was some sort of arachnid with an amethyst exoskeleton! that shit better make some good crafts). My squad arrived in time to dispatch their third forgotten beast in a single run, and the furnacer is recovering in my hospital, with his left upper leg missing, and his left foot apparently rotten, although I assume it's no longer attached to him - perhaps he's carrying it around?
All that is good fun, but since the first seige I haven't received a single dwarven migrant or caravan (only elves and humans). Perhaps the constant seiges and armies (the seiges include a coward general who runs away while I'm still dealing with the other squads) are scaring them away? OR perhaps the Mountainhome has fallen and we are the last ones standing. Interesting times...
edit: Well crap... now a bronze collossus has turned up :/ Time to improvise a hurried magma trap I guess!