Characterizing itself as a RPG, I think that Desktop Dungeons is more of a puzzle game than something else. The core concept of the game is fighting through a ton of dungeons, each time with new characters of various races and classes by killing enemies, finding gear and potions, using skills and unlocking thing for your city which will help you in the following runs. In practice, most of my runs were trainwrecks, I had few options I could actually use and the game felt like a puzzle that couldn't be won except by dumb luck.
The game labels itself as hard, but I feel that the difficulty isn't born out of fairness. Some puzzle levels are actually clever and they have a 'clear' solution. Like that one where you're a thief and you have to kill a bunch of enemies in a certain order to be able to kill the boss. Each class has special challenges for you to learn some of the tricks of the game and the tricks of that class as well, and that's fine, but the regular missions are also all about using the tricks of the game, and it doesn't feel as well thought. Are these levels beatable? Is there a critical path I should use in order to win - and everything else leads to death?
You see, there are a few enemy types and they have different abilities, you can easily kills enemies your level, but as the levels go higher, the odds get slimmer - the game is nice enough to show you how many hits it'll take to kill a foe and how many you can take. You can use items and potions to prolong your life and spells to damage enemies and buff yourself, but the point remains that a certain time in a level, if you have very low health and no more potions and all the enemies remaining can kill you in one hit, you've failed. Sometimes I had wondered, was it because I converted that piece of gear instead of that one? Did I not use my spells right?
You can convert gear and spells into bonuses based on your race, for humans it's 10% more damage, but other races like gnomes turn that into mana potions. You can also get attuned to a god to get special gear as long as you follow certain rules - although the only god I have seen was the god of Not-Using-Magic-And-Killing-Everything. This game is dense with mechanics and that actually feels like it works against it. I've lost countless times to situations where I felt like there was nothing I could do. I felt like it was actually impossible to win! I've wondered time after time "Is it possible to win this level? How does the game generate them? Does it create them randomly? Is there a critical path you HAVE to follow in order to win? In that case, what's the point of all the extra mechanics?" and just thinking constantly about that, as I lost over and over, kinda soured me on the game.
And ultimately, that's all I have to say. The times I won felt like I had used some death-defying magics to allow me to fight a few more turns and just brute force my way through the boss, and 90% of the time I had to try again. It becomes boring after a while, and that's why I stopped. Desktop Dungeons might interest you if you're ready to invest some serious time in it, maybe there's something I should've understood that I didn't, but oh well.