This game is a rogue-like dungeon-crawler.
Firstly, you have to choose between four different classes (Warrior, Wizard, Assassin, Shaman) and four difficulties (Easy, Normal, Hard, Hell).
Then the game starts, and you have to go through 7 floors killing monsters in order to get xp, money and items.
You will encounter bosses that are quite challenging to kill. Do not hesitate to escape, by closing the door that you opened a few seconds ago for instance, if you think that you will have a hard time killing a scary boss !
You will trigger quests (between 0-3 at each floor) that require to either kill a boss or find an item. You'll be rewarded with both money and xp. I think that you should clean an entire floor before triggering a quest (at least in Hard /Hell difficulties). The point I want to make is that if this is a boss quest, you might be in trouble if you have to fight a boss that you've just spawned plus some regular monsters.
Cleaning an entire floor will bring you everything you need : xp, money and items. You should get the best stuff you can and some potions before fighting the final boss. Regarding this final boss, I do not want to spoil people, but there is one thing I want to say : how is it possible to give him the ability to kill us with one skill and one shot ? If the skill "Death" hits you, you will die 10 turns later, no matter what you do ... You have to hope that it will fail :s
Regarding the achievements, there are quite easy to unlock, except maybe for the one that needs you to beat the game in Hell mode. I might make a guide to give you some hints.
To put it in a nutshell, I think this game is quite good, especially for its price (5€ at the moment). I hope that there will be some updates in order to add more contents (more floors, maybe a "survival" mode with infinite floors ? ...). If you like this kind of game, do not hesitate, buy it !