Chests

Normal chest
Chests are your bounty. They are your treasure. Opening them is the only way to gain permanent levels for your heroes. You get these chests by defeating the boss of a stage. It's not guaranteed you will receive one, but it is estimated you get about 1 chest for every 8.5 bosses you defeat.

Once a boss drops a chest it will be boosted for 5 minutes. As long as it's boosted, you will receive 6 items from it. When the timer runs out it will transform into a common chest. Opening a common chest will give you 3 items. Therefor it wise to only open these chests, if they are boosted.

When you open a chest a random hired hero will be selected and the chest will be emptied there. Depending on the state of the chests (boosted or common) 3 or 6 items will come out. There are 4 types of items you can receive from a chest and i will list them in most likely to rare. As long as you have at least one boosted chest, you will see a chest with a timer bar in the bottom right of your screen. Clicking it will give you the option to open it. That is, as long as you have keys. Should you have run out of keys, you'll be given the option to buy them.
 * 1) Basic items: Common. Any of all the items. These will increase your permanent levels.
 * 2) Skillpoints: Uncommon. The blue bottlecap shaped things. These skill-points only last for the duration of the run.
 * 3) Runestones: Uncommon. The purple things that you use to upgrade or craft artifacts with.
 * 4) Rings: Rare. Unlike the items, the rings that you can receive can be of any hero. Not just for the hero the chest is opened on.

If you no longer have any boosted chest, the chest in the bottom right will disappear. This does not mean you don't have any common chests anymore though. But how will you know?

Duarg
Click the MASTERS tab. Duarg is the first master. He is the one who guards both your keys and chests.

Here you can see how many chests you have left and also if you have the keys to open them.

Boosted chests will not all turn back to common chests once the timer runs out. Each chest has its own timer and the timer of another chest will not start until another one runs out.

The green circle with the lighting in it can be clicked. This will give you the possibility to boost them again. There are two options for that. Either watch an ad or spend gems.

In case you're wondering about forgetting to open chests. Should you want to time loop and still have left-over chests, which is very likely, the game will pop up with a message telling you that you have some things left that could be of use.

Mimic chests
coming soon ... hopefully