This sequel would not be complete without a wall based defense mode. At least in Unity this is rather easy by just setting up a wall consisting of multiple game objects with stationary rigid bodies and circle colliders.
The depth values of these pieces when live are automatic through a component that sets the z component of the local position.
This worked much better than expected. I just force the humans to always face right, only create enemies on the right side, and just let physics and basic AI do everything else.
I also added some other minor features, like reward boxes with randomized contents.
Moving right along....
If this first area of the game with these several characters for missions and weapons work out, then extending this experience through other areas and content will be rather straight forward.
I figure each area will have unique missions, but each will have common ones like exploration and wall-based defense with randomized rewards.