I'm not sure what the problem is for you, unless you have gaps in your workbench coverage that's letting enemies spawn there.
Workbenches should be providing coverage to EVERY part of your base inside the walls, which only allows enemies to spawn outside of the walls.
That's what the walls are for, to give you a first defense against event raids.
And no, the only predetermined spawn points are the actual, physical spawners that you'll find in Black Forest (Greydwarf spawners), Swamp (Draugr/Skele/Surtling), etc.

Edit: Forgot to mention that when you're raided, you should always move outside of your walls so as to prevent a section or 2 being broken and enemies flooding in. The protection they provide should give you enough time to react to a raid and move outside of the walls, pulling the enemies to concentrate on you instead of your base.