@rtsgamer or
@MsMuffins may be able to help
or check out the info and the links in
https://forums.projectceleste.com/threads/lmcg-legendary-meta-community-guide.4039/
It's a WIP by MsMuffins, but the links may be able to help with other build orders and setups. I can't really be of too much input myself cause I don't really play DoC, but I can suggest some more generic countering to seige:
1. get more LoS in your setup and pre-emptively split your units up, stagger and split cavs for an easier time getting small numbers onto different seige asap. If you're not at the edge of the map you need to be scouting out pushes and potential raids with units (while minimising damage and time away from army on the unit/s you scout with). Use a single cav to bait away infantry from the fight. Keep a squad of cavs or another strong unit vs seige (preferrably quick to snipe it and get out) off to the side of the push so it can come in uninterrupted on the flank.
2. get priests to convert and split them up between and away from other units, put them on their own control group/s for sure. Prioritize converting the seige and enemy priests first, then cavs, then everything else. If your vil numbers are where you want them, delete some (up to about 10% of vil pop) to free up conversion and/or military production space. You can always lose or delete the military and rebuild workers as long as it's not too many.
3. Get range bonus or damage gear/advisors on your own seige and rely on being already set with units out in front. It's almost impossible to win a seige v seige war by attacking into the opponent, you have to seige a choke and let them come to you, they have to harass you to death from other angles. Any damage and range advantage with items only increases the strength of being set up earlier with your seige line. If you're fighting right up at the edge of the map, back your seige (and everything else) up a tiny bit, with range bonus setups on seige/ranged so the enemy ballistae have to come to you.
4. depending on what else the ai or other player is throwing at you, you may be able to pull some villagers to defend vs seige while keeping their army and some of the seige targetting busy with your army. They'll do more than most ranged units would, and being melee force minimum range attack cancelling. This is only a good idea when you're in a good economic shape and can push back quickly but need to save important structures of your own (or the DoC wonder) in a close fight.