Your prisons will be more exciting thanks to permanent upgrades and characters that will bring new stats. You can spend some gems in buying the basic, advanced, or expert chests. 
They will randomly provide a certain number of fragments that will unlock and level up some of the permanent features. Some of them will only be available after a specific prison, but once you unlock a card, it will work in all your levels. 
To find these cards, tap on the card’s icon, right side icon on your playing screen. If you touch the different cards, you could see the different enhancements that they provide. There you will find a Get Cards button on the lower part of the screen.