My advice: don't play this game too much. You will end up bored in just a day or two and it will also become just frustrating grinding.
The game has only 6 maps, in which only 3 you get XP and souls for upgrades. And you will most probably play only 2 of those 3 cause the one named Unholy Sleep is terribly small, boring and stupidly hard compared to the other ones. All the other maps are pretty good imo, but Unholy Sleep is just badly designed. It's 90% narrow hallways to get trapped in by mobs, 8% just some rooms which look like torture chambers and underground graveyards and the same time and 2% dead ends. But that's it. All the other maps have more interesting elements like statues, trees on fire, lava, and big cave halls, but Unholy Sleep is so simplistic and hard, I'm sick of playing on it. When you finish exploring every map and everything else, the game is basically just farming souls for upgrades. And from then on, it just becomes something else on your phone you touched a few times, then never again or you uninstalled because it's not worth your time anymore.
The game looks really good for a mobile game and I love that. Same goes for the music. Maybe it's not as good looking as in the screenshots in the store, but it really is something when setting every option on Ultra.
Levelling up happens way too fast if you ask me. Reaching level 30 means nothing. And I don't get the Score Reduction that depends on the difficulty and on your level. Players already get more or fewer rewards depending on the difficulty, so why does it even exist?? It doesn't make any sense. It just pushes players who want to have fun and farm souls at the same time (and by that I mean most players) to play higher and more frustrating difficulties which are obviously not so fun. That while also pushing players to only play with weapons that have the most upgrades in order to be effective against high difficulty mobs - that just reduces, even more, the possibilities in which you can play the game and the diversity of it. (Cause having just 6 maps wasn't dull enough.)
Oh, and the upgrades? As soon as you're mostly done with the Achievements, they just become dummy hard to obtain because of how expensive they can get. This also adds up to the limitations of levelling up. If you want to use a different weapon you haven't spent any upgrades on as a high-level player you're better off starting all over.