Defender 2 Hints for Android
When you upgrade spells you will need more Mana to power them therefore you must remember to upgrade your Mana Research and the Mana Power in your Magic Tower.
Both of these will become essential as you progress up through the levels so unlock them early in the game.
Try not to waste too many crystals on upgrading the first batch of spells because the next tier replaces them and same again with the final tier. The ice spells are by far the most useful.
Save some crystals for new Phantom crossbow purchases as the bonuses are well worth getting.
By playing wisely it is possible to not have to use castle upgrades for walls until after level 20. This can be achieved by keeping the enemies at bay and investing in mana powers.