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 lose a life between around level 10 and level 80. Then I started losing about 5-10 lives each level at 80...probably would've died at about 90, but I saw the writing on the wall and gave up.)