Easy game, snipers are the good beginners, and personally, I find Ice Towers really useful. For first few levels electro towers are essential, later switch to ice towers. The levels seem to get way easier, the maps are less complex and overlap so towers get many opportunities to attack.
 "Frontier"){kind=icon}
 "Frontline Defense First Assault"){kind=icon}
 "Pirate Defense"){kind=icon}