This looks amazing for your first low-poly map, almost near nothing to what I first created for a low-poly map, though I have a suggestion or few as requested:
As far as everything looks good, the terrain kind of bland when it comes to the tone of colour. There’s not enough diversity when it comes to the colour of the terrain, as well as the trees, so I’d suggest adding a few different shades of green to give it uniqueness.
The same thing may be applicable to the border walls/hills around the map, though test with that and see if it fits.
You have nice terrain going on, but there’s barely if not any foliage in terms of grass, rocks, flowers etc. Having those little things will really pack a punch to your terrain build and make it look great.
Last thing I can recommend is testing out different lightings for your map, even though this one looks decent, I do feel there’s other lighting settings that can spice up your map entirely. Lighting plays a huge role when it comes to low-poly games.