Age of Empires II: Definitive Edition

This is the second mission of the Polish campaign (Jadwiga).

At the beginning, there are three neutral Polish cities. Each has a flagged area you can see--one each in the west, center, and east of the map. You can convert them to your side by building a castle in the flagged area, and Halych can do the same.

About half an hour into the game (by game time--the actual time depends on game speed), Halych will build a castle in the middle city, Rohatyn, if you haven't already taken it. As far as I can tell, this is scripted and instant--he sends a taunt message and the castle exists without having to be built (if anyone has seen elsewise, let me know in the comments and I'll update this solution). This counts as losing a neutral town, and will prevent you from getting the achievement.

This achievement is not hard, as long as you're aware of those requirements. I spent the match with only about four villagers consistently on stone, and three more on building duty once I was almost ready to build my first castle, so it's not going to take a huge focus or drain on your economy. You start off with 100 stone, with another 1050 between your town center and Lviv. Lviv will ask you to rebuild seven of their wall segments, and it's worth it for some free upgrades. Ultimately, that gives you enough stone for your first castle, and 465 out of the 650 you'll need for the second castle. For more stone, head south-southwest of your starting Folwark. There's another 1050 stone, more than enough for your second and third castle. Be careful, because it's guarded by a few wolves. They're easy for a group of villagers to take out, but if you task them to building a mining camp or directly to the stone, they'll be distracted and get picked off. If necessary, there are another five stone mines directly east of Rohatyn.

Halych will periodically attack your castles. If you're playing on standard difficulty, it's unlikely he'll ever actually be able to defeat them. In particular, if you've been following the Lviv questline, you'll automatically get the important Murder Hole technology. The achievement will pop at the end of the mission.

If you have anything to add, let me know. I'm happy to update my solution if there's anything wrong with it.