I'm playing the Apprentice, and I often am unsure how I should invest my mana during the build phase. The cost of creating new towers (40 mana in the case of Magic Missile) seems to be significantly less than the cost of upgrading an existing one (100 for the first level, and 200 for the second, for instance)
Should I focus on building new towers until I've maxed out my total Defense Units? Would 2 towers in close proximity do more damage than one upgraded tower in a single area?
Or will my towers grow significantly more powerful if I upgrade them to the second level at a cost of 100 Mana?