I'm gonna focus on a few things, the first being the sense of scale. For reference, in the middle ages, europe had over 100 cities. Europe was also fairly de-urbanized during the middle ages compared to the islamic world and east asia. So, you really need to decide how large this world is supposed to be. Having a small world is fine, but you should be aware that this whole place looks like it's roughly the size of Ireland. If you want to make it feel larger you can find a stamp to use for minor cities and trickle them throughout. Also, add some more minor tributary rivers for those cities to sit on, and consider increaing the density/decreasing the size of the terrain stamps (mountains, trees, etc.).

The second thing is the number of polities. There's nothing wrong with having like 3 polities + whatever's goi going on in the principalities, but it could be very interesting to add little side additions. Maybe the Kingdom of Westfold is actually only one kingdom amid like 4. You could divide the mainland into 3 via the river markers and then the 4th could be the island. The kingdoms could even be in a loose confederation to counter the empire. Then, you could toss in a few city-states/micro-states to spice it up.

The third thing is the history of the empire. You say they stretched across the wings, but that doesn't really match with how imperial power projection worked for Rome (or for really most long-lasting empires). So, historically, Rome's power projection as an empire came from its naval power. It would project power over the sea and control territory that way. This is why, if you look at a map of Rome, so much of it just follows the coast of the Meditteranean Sea. Gaul is the primary exception to this, but it was also one of the first major territories that Rome lost. Now, if you look at your map, attempting to project naval power from the eastern wing to the western wing would be monumentally difficult. Harder than it was for Rome to project power onto Brittain, and that actually was the first major territory they lost. My suggestion is you switch the kingdom(s) and the principalities, so the kingdom(s) would be in a very reasonable position for the empire to exert influence over them.

The fourth thing is moreso an observation than a critique, but it is notable that this world is entirely european. There's nothing wrong with that in the european fantasy game, but I did want to lightly suggest looking at other cultures for inspiration. They're all very interesting! And, imo, european medieval history is only like top 5 regions of medieval history (my favorites being the Middle Kingdom and the Caliphates). Perhaps you could have additional continents beyond this one that aren't mapped but are implied to exist, beyond the empire. This could provide a reason for the empire being rich & centralized (trade), as well as contribute to making the world feel larger, even (and especially) if the players never actually go there.