Please grant me a moment for a brief history lesson, it is important to understand the game. Once upon a time, mobile games, Apple and Android, received a free open source roguelike dungeon crawler called Pixel Dungeon (Also on Steam by the way) that brought proper roguelikes to mobile in a fairly elegant and popular format. Since it was open source, there were many, MANY mods created, even among the east asian spheres (Some of the Koreans and Japanese are making some neat stuff with it), but by miles the most popular of those mods is Shattered Pixel Dungeon, this game, and it is not hard to see why. The QoL features and elegance it adds is quite nice. And while it is so different from vanilla Pixel Dungeon as to be basically a completely different game (Let me be super clear, PD and SPD have so many mechanics different that you would be comparing Half-Life to FEAR when asking what is different between them.), Shattered is still overwhelmingly the most popular way to play Pixel Dungeon in the various Pixel Dungeon communities.
The game itself is fairly standard roguelike in the classical sense, but with a few important changes. Stats are limited, levels only grant accuracy, dodge, HP, and a few perk points for your skill tree. The enemies all have very video gamey effects, not quite Shiren/Mystery Dungeon tier, but you will usually need to use the in-built examine command to check the gimmicks of some enemies to play around them. But most crucially is this: Hit chances are fairly low for everyone. It is not unusual at all for enemies and player to have 6 rounds of miss, miss, miss, miss, even properly built with an accurate weapon. This is by design, because there exists a mechanic to guarantee hits by scoring a "surprise hit" by breaking line of sight. Even just backstepping through a door once is enough to give you a surprise hit even if the enemy was right next to you and fought you for the past 3 turns. This is crucial, and, hell, it is mostly your only counterplay to a lot of what the game throws at you. Healing is fairly slow and a limited resource, so guaranteeing hits where you can adds up and stacks the game in your favor, so a lot of your time is dancing enemies around 1 square obstructions or training them to doors. If you cannot abandon your bump combat habits to at least add steps to draw enemies back to doors or around corners, you cannot succeed at this game.
As a corollary, damage and defense is always in a range like 6-48, which means even an excellently defended character is prone to eating shit if they roll bad on their defense given the swingy ranges this game has. So minimizing risk and using positioning to avoid taking damage is important unless your class has additional defensive abilities.
The other biggest shakeup is that there is a very limited, but constant, set of upgrade items. Potions that make you permanently stronger to equip better gear, and scrolls that upgrade said gear. Knowing when to use the upgrade scrolls and on what is an art. Most of the time, holding them until you get a max tier or near-max tier weapon and armor and taking them both up is enough, but maybe taking that Ring of Evasion to +16 making you nearly invincible is a risky but winning move. Maybe upgrading your array of recharging wands is how you'd like to win. There are quite a few paths to winning depending on where and when you have to start spending your upgrade scrolls, and knowing how many you need to spend early vs saving for later is just as important as what you eventually end up spending them on. Most of the game's items are suitably impactful and very rarely do you come across total trash. And anything that IS trash can usually be fed into the game's crafting system as a component of a high-tier escape item or some other means of acquiring power through that system.
Each world ends on a boss which is extremely video gamey, complete with gimmick phases, attacks you have to carefully walk into the correct areas to avoid, and other patterns. Mercifully, they tend to be a bit easier than the enemies going up to them so in a lot of scenarios you can still win if you devoted some of your kit to avoiding the worst random monsters.
Despite starting as a mobile game, SPD provides a fairly elegant interface for keyboard controls and controllers, but life is still going to be a lot easier for you if you can bear to use the mouse occasionally. As a bonus, it is really easy to pick back up after a break as the game provides all of the information you need in the various in game help files and there is seldom complicated backtracking to deal with. Since each run is about 2-4 hours, you really would have issues doing it all in one play anyway unless you were one of the worst/coolest office workers in history.
The overall difficulty is, I would say, fairly easy as roguelikes go. It is generally tough and there are a lot of ways to lose a run early with bad luck, but I feel most of my runs are winnable and even I, not a particularly great player, could get to a point where I reliably streak wins once I understand the game more. For people who want to do that, the game offers a series of challenges that can be selected a la carte, or in any combination you like, to make the game harder. Tougher bosses with more gimmicks, less food, fewer upgrades, if you really want the game to beat you down, you have these modes to really make you understand how all the game's various tools can be used.
As a fairly simple and elegant roguelike that still captures the feeling and variety of classic roguelikes, Shattered Pixel Dungeon has come out of nowhere to be one of my favorite roguelikes in recent memory. What it might not have in breadth, it has in ease of play. Even among the far stiffer competition of PC roguelikes, Shattered Pixel Dungeon is still hard to beat.