Hard but not impossible
This was great. I was about to dismiss it as impossible but kept trying different tactics. I finally was able to complete all levels. My only gripe is that the levels actually get easier after the first one because of the added starting money. Btw, my main tactic was to place a bunker near the start and then a cannon just as the bunker accumulated enough money for it. After that, I'd upgrade my bunker or cannon or add a second cannon (depending on the level). At first I didn't like the artillery units, but fully upgraded they are impressive.