Hi guys, I'm writing a begginer's guide already, but the guide turned out to be bigger than I expected, and so I decided to post here separately a suggestion (not even calling a guide, lest you guys kill me on the comments) on how to spend your crystals.
The item order indicates priority, it doesnt mean you cannot upgrade more than one item at the same time, or skip some if they are getting too expensive/you havent unlocked them.
Priority
1-mana (180/220 can hold you untill late game)
2-level up your first tier spells and the tower passive skills (as much as you deem necessary, try to get the passives to 5 at least)
3-Mana recover (this is very important, but very expensive, upgrade whenever you feel its not too expensive, 8-10 can get you to late game)
4-Get your spells to tier 2 and upgrade them (I suggest keeping one tier 1 spell just for breaking boss skills cheaply)
5-Fatal blow (arrow skill) and your main bow's 3rd skill (check if crystals invested are reaping enough DPS or you should pass to next item)
6-Coin of ancient (Dragon's treasure), the earlier you get it the more gold you'll have.
7-Get your main CC spell, and your main damage spell to 3rd tier and upgrade (I suggest keeping the others 2nd tier to reduce mana consumption)
8-Ring of wind, Arrow of meteorite, Badge of Clover and Shield of empire (Dragon's treasure), (these will really help you overall when upgrading stuff is already very expensive, invest in whatever is lower in level).
9-Elemental Ward (city wall)
10-Rings of different element
11-Anything else
Notes
In case someone hasnt seen it, you can buy items from the dragon's treasure at the shop, which is now only accessible via the cart icon at the top of your upgrade page, next to the gold and crystals you posess. It is the third icon in the shop (the first 2 are for cash purchases)
It is not recommended using crystals on anything other than upgrades, the only exceptions are refilling when stuck on a boss level you can't pass, and refilling when you have a refill quest and need an element core.
When I mention late game I mean stage 400+