You can get up to level 18, playing the later maps in dark forest. You get experience for creatures up to 3 levels below you (unless at the current level cap, 45 as of this posting).

Campaign level ranges are currently:

Forest Haven (1-10)
Dark Forest (11 - 15)
Balefort Castle (16 - 20)
Fathom Crypt (21 - 25)
Lost Expedition (26-30)
Ancient Swamps, and Return of Skeller (31-35)
Alien Oasis I (36-40)
Alien Oasis II (41-45)