Letter Quest: Grimm's Journey Remastered

Upgrades only count in Story mode, even though you can buy attack and defense upgrades in Endless mode. More types of upgrades will unlock as you progress through Story mode, and each upgrade for a specific item costs more than the previous. Because of this, you'll need a lot of gems to purchase 30 upgrades (around 20,000 or so).

It's highly recommended to purchase the upgrades for increasing the gems you get from gem piles first, as this speeds up collecting gems in general, allowing you to purchase upgrades sooner.