I have a idea for the next update you can make the training have a mini game for 2x exp?
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It's a pretty cool idea. Probably not worth the time investment but I'll think it over. Also I see you've been commenting quite often here, I've got a discord server that you can join if you'd like to share more ideas or chat about the game with other people, it's probably a little bit more convenient than these comments, the link is on the game's main page and in the credits.
I have something similar planned for stat training locations. Gold in the game is kind of in a weird spot right now where it's almost worthless, so I'm going to be adding an permanent upgrade system to the stat training spots with gold, one of those. One of these new upgrades could be that you can spend a certain amount essence and increase the training amount by a proportional amount, for example you could spend 15 essence to increase the stat exp by 150%.