Am I not getting the point? I mean I see you're only learning how the mechanics work, but - you built the bridge with a gap so nothing would be able to get in. So where's the problem with the gates you mentioned? Anyways, again: Yes, terraforming and maybe block line of sight etc. I'm not going to spoiler now but you can never have a 100% attack-safe base if it's not a small island in the middle of the ocean. You'll most likely always need to do a bit of repair-work.
And with a bigger base/building, you cannot have 100% protection from archers unless it's an island like mentioned above. But an island will only do it's purpose if there's no other land around where something might spawn.
Last thing that might work would be to plaster the whole area around the base with base-items like the WB etc. but ... yeah, sounds silly and actually probably is. You would need to cover more than the actual shown radius on the minimap. Which is quite large actually.