Unused energy should roll-over like unused minutes. Either that, or converted into something useful. It takes a lot of time to regenerate, and it is unfair I believe to have to lose them when I level up. Great game though nonetheless.
I know these games aren't meant to be beat so easily, but this one seems extremely difficult to advance and gain gold. Maybe i've been spoiled by some of these others that allow for much easier access to upgrades. Fun game nonetheless.
 "Clash of the Dragons"){kind=icon}
 "The Final Death Wish"){kind=icon}