Interesting concept, figured out late into my first run that towers need both resources to operate. I think you have the right idea including the bow and cog symbols below the bars, but you could probably make them a fair bit bigger as they are easy to miss. The range of towers seems a bit small as some enemies seemed to just beeline towards the doors and never come into range of them. Overall there are some good visual design choices, e.g. the waggon icon being filled in radially to indicate when the next waggon will spawn is a nice touch.