Island bases are safe as long as everything is covered by no-spawn items. (list here: https://valheim.fandom.com/wiki/Player_base).
But so are mainland bases with a simple terrain ditch or wall.
Even stone walls work well as any piece of stone wall has 1.5k health and trolls only do 50-100 damage per hit. (This means while more expensive a wall made of smaller stone blocks is more durable). Just don't build anything right against the stone wall as trolls attacks can do area damage. You don't even need a solid wall, spaced 2 block high pillars will even stop trolls due to.their size.