Earlier quoted context omitted.
You can keep playing after you finish. It seems to get much harder around level 80, with multiple orange dudes, and you end up losing multiple lives a level unless you've strategically placed your orange and purple turrets. (I finished the game with 91 life remaining - I screwed up a little when the first new path was added, didn't expect it and didn't have enough money banked to build adequate defenses - but didn't…
You made me curious so I played it out... http://i.imgur.com/Y2M6j.png At some point you just become unkillable. edit It's still going ;_; when will it end?! I wasted 2 hours on this already.
How did you do that? I stopped receiving money completely around level 80 (no more cash to buy or upgrade!), and very little money since level 50.
From then on, it became increasingly difficult. I only got to level 141 (Buckingham Palace).