If it doesn't have wings, it won't fly.

Defense in depth. A low wall over which trolls cannot step constructed in multiple layers can be far more economical barrier than a high wall. You'll still want at least 2-3m wall for cover from ranged attacks, but that can be placed behind the outermost walls.

I still haven't seen a single seeker actually try attack my home yet alone try crossing the walls, but at least bats seem to be capable of that. There isn't much of a counter to them other than using the ballista. There may be some way of using the sharpened stakes to fend off seekers trying to fly over walls, not sure.