If you want to play this game offline and without any browsers after December 2020, then download .swf game file and use any Flash Player that runs .swf files.
Another way to play flash games is to install SuperNova SWF Extension (it works in Chrome only).
Other than the final boss selling your towers and me having to do that level 10 times over, it was very fun.
This is a very fun game. I could not buy the heroes. Three were free. It would not let me buy any. :(