There is only one solution:
MAGMAAAAAAAAAAAAAAAAAAA!!!!
I have no access to my magma, at present. Otherwise, I would have done that already.
dam you that was my idea
anyway, you could just change the strenght of the orcs or use multikilling tools like death bridge and drop them 10zlevels and maybe crush them with a drawbridge, thats the easiest at start.
i also have this problem so youre not alone..
I can't get enough materials to make such a thing right now, being besieged. I'm trying very hard not to cheat and use my Free Wood reaction to build this tower more quickly.
So the Orcs can't get in, right?
That means they've set up camp. Right?
Tunnel under the bowmen and collapse them. Climb the walls and shoot the rest.
I can't tunnel, as I have no picks. This is one of my challenge games - I cannot break earth, and can only use glass to construct the tower. I also have no metal or ores to use to make picks, were I to decide on that.
I've installed the orc mod, and found that, rather than half the year, the sieges tend to wait about 11 months - or right until the next siege shows up.
...
there are more than a dozen orcish bowmen outside, including a legendary one, all of them with steel arrows and various bows.
So, from the second part I can tell that you've modified the orcs somewhat, since they don't show up with steel arrows normally... But they would if you gave them [METAL_PREF]. Have you made any other modifications to them? I ask because getting them to consistently show up to siege when I gave them more active seasons was a large problem, but if you've solved that I'd like to try to get it into the next version. Probably in a 9-months-on fashion, rather than year-round, but more than the current 50%.
The orcs will ramp up based on the number of previous sieges, not wealth or immigrants, and that's unfortunately hardcoded.
As to dealing with them, I'd suggest a corridor angled to prevent the snipers from firing through it filled with spikes (green glass if nothing else, since you seem to have that). It will need to be fairly long, but it's one of the few consistent ways to deal with orcs without a large and well-trained military.
I gotta say, I'm not really a modder. All I did was change them as you'd originally suggested in another thread to make them always at war with me (I wasn't thrilled at the orcish caravans, remember). I didn't add anything else. These orcs are otherwise unmodified, and I think, technically, they leave about one month before the next siege.
Also - I can't construct anything more until they leave, since my entire fortress is a 20x20 glass wall square, with an 18x18 room inside and a few parts of the next level's floor installed so I could build beds. I'm great on food and such, but wood and other such resources are about out and I hadn't gained access to my magma (thankfully exposed, but not close enough to run-build an extension to).
Finally, as a question to you - is there any way you know of to make invaders hostile to (or enemies of) wild animals? I would think the fire imps wouldn't be any more thrilled with a dozen orcs at their pipe than they would be a dozen dwarves.