OK, included here are 2 castles, I will discuss why 2.
A special thing about the castle base type, is it hides the units inside. So in a way it is more like a city that hides what's inside.
After doing art and testing, I discovered that this makes a user experience issue!
1. The art can be like other bases, and show empty interior so you can see the terrain. But then, it's very confusing because it looks like an unoccupied base, when really it's occupied but you can't see what's in it.
2. The art can look more like a city (or big castle), and it's obvious you can't see inside because it looks closed behind walls. But then the problem is, it's just like a large size 15 city, and it's hard to see the terrain under it without middle-click to get tile info.
So, then I did a compromise. I went back to the art for #1, and made it "hint" a little more that you can't see inside, while still getting a tiny hint what the terrain might be (but not much.) And so now it is up to ruleset designer to pick between the pros and cons for a base that hides the units inside:
Q. Do you want to see the terrain but the castle looks empty? Or do you want to not see the terrain but it's very clear you can't see who is inside and that it might not be empty?
A: You get to choose.
Also included is a quick unit activity sprite I did for making castle.
1. unit activity icon, bottom row , position 3:
2. Castle sprites, from left to right
Example of closed castle on map. As you see, very little idea what terrain it's on:
Example of open castle on map. This one is on a mountain, which you can mostly see:
These and some other much more exciting things, will be in the launch of the Multiplayer2-Caravel ruleset coming in March 2021 at https://www.freecivweb.org
FCW Community chat server:https://discord.gg/g2cGjTV