How about having on start small odin city which can build only blasters, colletors, relays and storage and for specific number of enegry it will uprgrade -> so after that you could use mortar, speed and SAM, in the 3rd level you´ll can build drones and on the last level you´ll can build Thor ...
I rely that on first levels will be some of levels of city locked ...
But then whats the point in having the schematics? some maps you can get the totems earlier but to make it more of a challenge you need to the schematics.
No, that's why we have techs. ;D