How do you create obstacles in your own arena?

So, doesn't the tool that removes pathfinding on the global map (the yellow one) work?

Is it possible to make unique city buildings as separate buildings?

I'm 99% sure that it's not possible. An ID is just a number that indicates an element in some array. You can't just take an ID related to the array of city buildings and use it to index the array of buildings on the map.

How do you change the stats of an artifact or create your own artifact or set?

You can change the simplest parameters of artifacts (name/description/icon/cost/bonus to primary stats or morale/luck) in the GameMechanics\RefTables\Artifacts.xdb file. Of course, this will result in a mod. As far as I know, creating your own artifact is only possible by replacing one of the unused artifacts in the game (e.g., ARTIFACT_PRINCESS)