Originally posted by GunsForBucks:

You also want to clear the area of smaller trees and brush. Enemies will not spawn in line of sight. So clearing things that block vision will prevent them spawning as well.

Mobs spawns dont work like that in valheim , world spawns works like big area squares ( 64mx64m ) , trees or line of sight dont affect them
Here is the souce
https://www.google.com/amp/s/amp.reddit.com/r/valheim/comments/n3pn4g/enemy_spawning_explained/