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.
Fatal Blow and Multiple Arrows
Both of these will become essential as you progress up through the levels so unlock them early in the game.
Mana
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.
Phantom Crossbow
Save some crystals for new Phantom crossbow purchases as the bonuses are well worth getting.
Spells
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.
Stage 900 (Boss Fight)
Check out the video below for some hints and tips on playing the stage 900 boss fight.
Upgrades
During the beginning of the game try and progress as far as you can without upgrades as this will both build and bolster your in-game currency, and make you a better player.
Castle Upgrades...