A little suggestion, if I may. I think it'd be a really good idea to add in something that will tell you the current time it takes to b able to cast a spell, separate from the bar. A line that says, "Each cast will cost (how long it takes to acquire energy total" or something to that effect.
An AI system added to this game would make it probably one of the most fun things I've ever played, namely for making entities attack each other. Maybe a few basic things, like walking, attacking, jumping, etc.
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