basically u gotta build machines to stop the wave of monster things. u can upgrade ur weapons and place them wherever u want. you will get the hang of it. its very fun and ther are 50 levels, i have to go now so i only got up to lvl 34.
HEres the link and how far i got to. ill play it fully tommorow
http://www.candystand.com/uk/play.do?id=18047

Lol i was addicted to the 'Bloons' version of this game for aaages lol, gonna have a go on this one now..

Wahh got up to lvl 30 that time with a score of
1392057