You get one free chest with cards and elixirs every 4 hours, and if you’re lucky, it might be an iron chest with rare cards. So don’t forget to check the store regularly! You can get more chests by watching an ad.

 

You can notice that sometimes the button is labelled "Open Now" instead of "Free." This means you can skip the timer and open the next chest immediately. The best use of this is when there is a lot of time remaining before the next free chest will be available (as close to 4 hours as possible).
So, the best strategy would be to wait for the remaining time (if it's much less than 4 hours) to obtain the Free Chest, and then use the Ad "Open Now" to immediately get another one (skipping 4 hours instead of the initial time left).

 

You can double the income from your shafts and forge for 3 hours by watching a small ad. To do that, tap on the “boost” button on the left of your main mine screen.