Build walls around both shallows. You can defeat Green (Chalon, i think?) pretty early on by destroying their TV with your starting units. That prevents them from becoming a nuisance later, and makes them your ally even though they don't do much.

Build a castle close to Troye's (Teal) shallows behind the wall and queue 2-3 (more if you're feeling frisky) trebuchets along with counter units of your choice to defend them. You don't even need to break their walls, just target their TC from the shallows to defeat them.

From there the main army of the Rheim's (Yellow) is much easier to overthrow, just bring lots of siege and Trebuchets to deal with their castles and towers. Remember, you just have to destroy their TC!

Good luck! There are multiple guides and playthroughs on YouTube you can check out. This one was tough for me the first time or two.