This app was mentioned in 3 comments, with an average of 3.67 upvotes
/r/Buriedbornes is my current timesink, a roguelike with optional PVP and interesting progressive build system. It's from a Japanese developer and any translations are crowdsourced but it's still highly enjoyable.
Used to play Dweller a lot, also a roguelike.
Lots of fun with Knights of Pen and Paper and its sequel. It's just the right amount of balance between meta and fantasy. More turn-based and strategic with a linear story framework.
Pixel Dungeon and its variants. I play the original. It's, yes, a challenging roguelike.
<em>POWDER</em> was originally developed for Gameboy Advance and then ported to other systems, so it's very light on resource requirements.
You may also want to play <em>Dweller</em> (google play link), which was originally made for the old Java2ME phones that existed before iOS and Android, so it should also run on very low specs...
I'm not seeing Dweller mentioned, so I'm mentioning. It's free, as well.