So, do tough enemies invade the beginner biomes? I was hoping I could have a safe base just on the Meadows side of the Meadows-Black Forest border. Or is this too early to think about a proper base? I need something a bit bigger than my little hut now.
I have my home in the meadows, quite some way from the black forest. Initially, I got raid events from meadows mobs - boars and greylings. The raid event warning message mentioned grass. Usually while I was at home. After defeating the first boss, the raid events changed to black forest mobs - greydwarfs, greydwarf brutes and greydwarf shamen. At my base, in the meadows. Quite a distance from the border. The raid event warning message mentioned forest. So I think that the raid events are based on the highest level boss mob you killed rather than on where your base is. But killing the first boss is important because it unlocks the use of pickaxes, which is a crucial step towards unlocking the use of bronze, which gives you reliable access to different grades of wood, which unlocks a lot of things to craft. Pickaxes are also a big step in building because it makes it possible to greatly change the terrain.
Other than raid events, mobs
can leave their biome and enter a neighbouring one to at least some distance. They definitely do it while chasing you and once or twice I've encountered an out-of-biome mob who was already there. Close to the border, though. My guess is that they wander within a distance from their spawn point. Mobs sometimes chase other mobs, so it might happen that way too. Biome borders aren't hard borders for anything mobile.
If you need a base bigger than what you have, it's time for a bigger base. But it won't be its final form. You won't be able to build in stone yet (that requires iron, which doesn't exist in either meadow or black forest biomes), so sooner or later you'll be rebuilding again.
My plan is to build up more before killing the second boss and thus making the raids worse. Better kit, higher skills. And more building. With stockpiling of materials. I only have ~1000 stone stored so far and I'll probably use at least that much just landscaping the area for my castle.