Technically…
I’m not aware that lights are effective against undead in this game 
Maybe one day, as there was something similar in the Candledark mod, but people is always biased with their knowledge from other games / lore so there’s always these kind of rumors.
say you build roads in the mined our areas. Will the goblins and invaders still spawn?
I’d say “why not” 
If we’ve had goblin camps spawning on top of your town, destroying your houses and walls, I don’t see why entities couldn’t spawn on top of roads. I don’t remember right now the current spawn range, so I can’t tell you for sure…
There’s only one way to know it 
Issue is just like Crab stated, we don’t actually build “buildings” to the game as we are not using the construction interface in a regular way were we would add floor and walls.
Maybe we should just have the whole mine re-floored?
Much testing needs to be done …
Last time I went for it undead killed just goblin chieftain, goblin minions turned invulerable and begun building multiple fires inside the caves making some rooms unreachable and messing with the whole design…
Who said goblins were no trolls?
Hah!
On the other hand as I get more Herathlings (+15) interaction between them tends to worsen, for example refusing to mine or do any work at all. I guess I will be limiting food supplies in order to avoid residential growth in the future.
or you could always just deny the new guys from coming… 
I was unsure if the decline option is working, have never used it in the past, but if it does of course I shall proceed.
t should be working, i havent seen anyone say that it doesnt.
I think if you build floors, you won’t be able to make storages for it
if you make floors you can, but if you make roads you cant place storage on top.
I dont’ mind the undead spawning in my mines, it’s at least reasonable. But I just had a pack of wolves spawn inside my mine. At least limit what enemy times can spawn in mines for now. Then add more complex systems later.