It is possible AFAIK.
Check out guides on steam that cover modding. If you can't find it, ask on discord

In a nutshell: Yes!

So this has been a bit of an issue, because there's technically many ways to do it but they require a lot of experience in coding, and chances are if you're messing around with models, a lot of people who'd want to do this are artists who understandably don't know much about coding.

For this reason I decided to write a library to drastically reduce the amount of coding and am currently writing a plugin for the Unity Editor (Game Engine used to make the game) that will allow you to replace models with no coding necessary at all, although that's not complete yet.

Neither the library nor the plugin are public yet, however I have been searching for some artists to test them, I'll write all the code for anyone who wants to test it out, keep in mind it will be glitchy but it has been used in a publicly released mod before (Skchuch's Day-Night Cycle - added lights to the buildings). If you're inerested, contact me on discord at @Agentfox#7515 or join the Kingdoms and Castles discord and drop a message in #game-modification, I'm pretty active there.

Invite link to KC Discord:
https://discord.gg/rsjgr6kz28

Originally posted by Agentfox:

In a nutshell: Yes!

So this has been a bit of an issue, because there's technically many ways to do it but they require a lot of experience in coding, and chances are if you're messing around with models, a lot of people who'd want to do this are artists who understandably don't know much about coding.

For this reason I decided to write a library to drastically reduce the amount of coding and am currently writing a plugin for the Unity Editor (Game Engine used to make the game) that will allow you to replace models with no coding necessary at all, although that's not complete yet.

Neither the library nor the plugin are public yet, however I have been searching for some artists to test them, I'll write all the code for anyone who wants to test it out, keep in mind it will be glitchy but it has been used in a publicly released mod before (Skchuch's Day-Night Cycle - added lights to the buildings). If you're inerested, contact me on discord at @Agentfox#7515 or join the Kingdoms and Castles discord and drop a message in #game-modification, I'm pretty active there.

Invite link to KC Discord:
https://discord.gg/jpGbaEHn

Is it possible to use premade unity assets that are free download?

yes although a few things have to be modelled to specifications, for example a road has multiple models for the different possibilities for connections, and a ballista has many small pieces that make up the full model