'Coin Dungeon' is very useful content on Coin Dragon Master.
You can play Coin Dungeon after the cleared 1-3 stage.

When you enter Coin Dungeon, you need key.
Normally, we give 3 keys everyday.
And the number of key is reset everyday on 00:00(UTC-0).

You can purchase more Key on Daily Shop.
But Key is randomly displayed on Daily Shop.
So sometimes you can;t purchase Key.

Coin Dungeon has 15 stages.
And when you go further, you can get more coins.