Originally posted by Mharr:

It'll be a day of glorious chaos if they ever gift trolls the ability to mantle.

or breed

just store your trolls in a hole my lads

https://steamcommunity.com/sharedfiles/filedetails/?id=2649696200

It'll be a day of glorious chaos if they ever gift trolls the ability to mantle.

The buffer zone is correct, any animal pen or garden right up against the first wall or fence is just asking to be wiped out by something with an area attack. Build a moat and wall or raise your whole compound on a plateau or live out in the sea.

Eventually there will be flying threats.
Also a good idea to put a roof over them.

I have a good experiences with moat arround my base - it works against both trolls. But it must be far enough from first wall. Or raise the wall with hoe - it also works and is undestructable. Or combine both ...

When I mine copper and have a big hole in the ground the trolls just stand on the edge looking at me, but never attack. Maybe a dig a pig pit?

From my experience, I'd recommend this:
1 - build the pen as large as you need and surround it with a fence. place crafting stations within the pen so, that their bounderies cover the entire area.
2- Create a buffer zone of 8-10 meters (4-5 2x2 floor planks) out from your pen - This should be far enough that the log trolls cannot bludgeon your boars to death
3 - dig a trench just outside your buffer zone, trapping enemies. So when, enemies are near and the pigs run around scared, your boars will be safe within their pen.

I always did a trench all the way around my base/building area. keep the sides steep as possible and spaced back away from any structures. they can't hit anything and just stand there getting shot by you with a bow.

I have my boar pens in a Meadows biome far from my base and teleport to it. The key is to not have too many items that establish player base too close together to trigger events that would endanger your livestock. So, no events, no problems.

Also I have a stakewall perimeter fence around the pens 10m out from the edge of each pen so even greydwarves walking by can't get close enough to see or spook the boars. It's as peaceful as you can get.

If you're not familiar with which items establish a player base, review this:
https://valheim.fandom.com/wiki/Player_base

Just keep items on that list far enough away from each other as to not trigger events near your boar pen and you'll have a much easier time breeding your pets.