Orcs are... well, intentionally difficult. Personally I'm in a similar situation - goblins too whimpy, but I'm not a big military guy so orcs are perhaps a bit extreme. They start sieging when your wealth reaches some unspecified level... as early as the first summer in extreme cases, the first winter in more typical skilled player cases. If you take wealth-increase slowly they take longer to arrive.
Version one: non-traditional traps. My most recent fort went a sort of "cheap" way of it - a set of repeating drawbridges over a major river as the only way into the fort. Drawbridges go down, crushing any orcs standing on the lip. Orcs path over drawbridge and start walking. Drawbridges go up, dumping any orcs on them into the water. Rinse and repeat. I think I had one orc get all the way over so far, and that because I had everyone busy when a siege came so it took a while to pull the lever. Orcs are immune to "traditional" traps like cages and weapon traps, but not to drowning, burning, dumping traps, cave-in traps and the like.
Version two: military, hard and strong. Draft a whole bunch of people, then give them crossbows with lots of practice bolts and sufficient high-quality battle bolts. Machine-gun speed death bolts blotting out the sky from behind fortifications is nice, with some melee for mop-up. Throw in a few ballista and catapult as well for good measure.
Version three: edit the creature_orc.txt file. Reduce their SIZE and/or DAMBLOCK. Reduce their litter size. Take away trAPAVOID from them and their mounts. Reduce the damage of their attacks. Edit entity_orc.txt and stop them using bows (and crossbows if they're still in there) or mod bows to deal blunt damage (and less of it).
Version four: lose the orcs, buff the goblins. There are a few "better goblin" mods out there, at least one with several difficulties (from "traditional" to "fire throwing death machines"). Or edit them yourself. SIZE, DAMBLOCK, etc.