>>299454253
We have a bunch of experience on this here. Map makers abound from /vrg/.
Basically what you want to do is setup good occlusion terrains or occlusion walls. That's the best way of doing this as well as flagging as Static everything that you know won't be moving. That alone will save you a lot of performance.

By occlusion terrain or blockers basically any collider mesh that would block out most other things. Imagine hills separating the two towns or place. Imagine a big wall separating one side and the other.
When you bake Occlusion that will make it so that you don't really render what's on the other side since it's not in the view of your camera FOV.
Another trick is to bake your lights and have no active dynamic light source when possible.

A more advanced form of frame saving is LOD or Load On Distance. This feature is something you can use by setting up lower poly meshes of the same thing that will load depending on the distance. This is basically all of those mesh pieces that you chuck into the same prefab that progressively goes down in detail the farther you are from it.