I feel as if the timer sometimes cuts long-term plans short, also, I think that ALL of the units should be upgradeable, for instance, the undead unit with the flail, or the ogre.
i maxed regular arrow and just left it to autofire right next to their castle on lvl 7
i left the game for a few hours and got like 500 000 xp u have to have a turret too tho and a trap