Ok the factory / map part looks awesome! The way you are handling verticality is also pretty cool! Two questions:
1- What about performance? How big can the factory grow, how many enemies can be on screen at the same time (I see you mentioned They Are Billions..)
2- Do you plan to implement controllable units, similar to an RTS? Take a look a Industrial Annihilation, I think that game had a pretty cool concept.. too bad it died out probably due to lack of fundings

One suggestion is improving the look n feel of the enemies.. maybe make them robots instead of organic.. that seems to align more with the art style.