Help with mighty empires

A few cheap suggestions:

For forests the Woodland Scenics clump foliage works very well, most people use it for bushes on regular bases, on a tile it would make a good forest.  I glue it down with any white PVA glue, then once that dries you can wet them with rubbing alcohol and then apply a liberal coating of PVA diluted 50-50 with water.  Once it dries the clumps won't go anywhere, they'll be locked down for all of time and are very robust.

For rivers you can just paint a blue line and put some gloss varnish over it.

For mountains it would be a bit more work, but Woodland Scenics makes rock molds for Model Railroading, and if you get some selections of those, preferably ones with sharp and jagged edges then Woodland Scenics also sells Lightweight Hydrocal which is a plaster that you use for the molds.  There are plenty of resources online for using them for model railroading, once the molds have dried you can pop the rocks out and break them up to make mountains, paint them and add a bit of flock and they should look reasonably good.

Swamps can be made by building up a small rim around the tile with some flock and a few islands then pouring a tiny amount of water effects, again Woodland Scenics has good options, and letting that dry, make sure to paint the bottom of the base some kind of green-brown color and you'll get a marshy-region that has shiny water.

If you want to make fields, building up rows of PVA next to each other can look like a plowed field once you flock it.  Cutting up a welcome mat also makes great wheat fields, but scaling it down could get difficult.

I think there are a few scans for 3D printed tiny castles and things like it, but not sure, unfortunately I don't really have any suggestions for buildings and castles.