Scrap coins, you'll click 3 materials at the bottom of the screen to gather in order to reach the material requirements for an upgrade on how much per click or per second and when you buy all upgrades, the game will prompt you with an end screen saying "You failed at breaking the city". idk that's all that came to mind, tell me if i could simplify it.
That sounds simple enough. you could set it up so that you have 3 buttons (they can be images too, dont have to be the square colored things) and when you click on it, you get X of the specific resource. you could add a few upgrades that cost the resources like a "+20% more wood" or "each click gives 1 more of all resources" or maybe some idle upgrades that let you get the resource slowly without clicking and then the end screen pops up either when you reach a specific amount of resources (checking that on each frame is bad performance wise. it wont matter for small projects like this but it is worth it to check that for example every time you gain some resource because it will be like once every second instead of 60 times a second) or if you buy the last upgrade (check inside the upgrades logic).