Hello :) I'd say your first attempt is pretty good. I did two playthroughs, the first I got defeated and the second I had victory.

Gameplay- The first time I played through, I didn't really know what I was doing or what was happening when the other kingdoms were having their turns. But after some time I figured it out. I think some things could be explained so the player doesn't get confused, such as the numbers next to the units and what makes them increase/decrease. Also, this is just my opinion but I feel like the random distribution of the reserves is kind of inconvenient sometimes? It's alright when my territory is small, but when it gets larger it seems to take a long time to move them across the map if needed (since you can only go one square at a time).

Graphics- Very simple, definitely works

Bugs- I haven't found any, but one thing I'd like to suggest is showing the victory text as soon as all castles have been captured and all enemies have been defeated. With nothing left on the screen, the player might be confused on what they should do (I had a moment like this in a Fire Emblem game and I was definitely confused when that had happened lol).

Strategic depth- I'm curious on the probability of the player unit defeating an enemy unit. Is it a 50/50? Also, one unit can take out multiple enemies in its turn, which makes the game feel more on the easier side. The game also gets significantly easier when a castle gets captured, since no more enemy units from that kingdom will spawn in. But overall this is a pretty decent game, keep it up :)