There are 17 accomplishments to achieve. Each one unlocks content.

Accomplishment	How to unlock
insane mode	clear the game on normal or hard
jukebox tracks 1-6	get 10000 points
jukebox tracks 13-16	get 25000 points
jukebox tracks 17-20	get 50000 points
jukebox tracks 21-24	get 75000 points
jukebox tracks 7-12	get 15000 points
new costume	get 5 gold ranks
new costume	get 40 gold ranks
new costume	get 30 gold ranks
new costume	get 20 gold ranks
new costume	get 10 gold ranks
new page in the gallery	never lose a tower
new page in the gallery	stomp 25 bugs in a level
new page in the gallery	kill a boss in under 2 minutes
new page in the gallery	only use pea shooter (don't upgrade or stomp)
new page in the gallery	kill all scorpions in a level
new page in the gallery	defeat a centipede before it burrows