Heya, I'm a new player so I might get this all wrong...but, it seems that you are limited to just ~2 hours of gameplay a day because of the energy.
After a few dungeon runs you will probably have some deaths (read, revives that costs energy) and also you pay energy to use the elevator.
This makes me wonder why it's called a Free 2 Play game, it's more like a Shareware that wants your cash or else it shuts down after a few hours, to re-open on a time limit.
This pretty much opens up for a HUGE amount of players getting VERY disappointed in the game early on, because of its limits.
As you save up on crowns, you can buy more energy from the exchange for around 2k per 100 energy. Or you can buy even more with real life money.