I'm not quite sure how to describe this game, because calling it an "inventory management autobattler" doesn't do it justice.
After you've picked one of the 4 classes and one of the two inventory config options for your chosen class, you end up in the shop. That's where the bulk of the game happens. You spend the gold you are given to buy various items to take to your next fight. Then you spend time arranging every healing herb, frying pan, strength potion, vampiric sword and lucky pig that you have purchased inside your limited bag space. This is even harder than it looks, as some items need other ones of a specific type placed in their vicinity for their special effects to trigger, while others can combine with one or two other specific items to make a stronger one, but only if they spend at least one fight next to each other in the bag. Additional inventory slots do appear in the shop, but not every round, so you need to be quite strategic. Once every 4 rounds, you are offered powerful unique items that either let you specialise in your class or simply give you more stats, but you can only pick one, so again, choose wisely.
Then you go to battle. This can be quite relaxing, actually - you just sit there, watching the health and stamina bars, and hoping that your opponent's numbers go down faster than yours. The feeling of smug superiority when you win is amazing! And even if you lose, you have quite a generous allowance of hearts before you lose the entire run. After the fight is over, you get more money and a better chance for more powerful items in the shop. The run goes on for 10 rounds (plus a few optional bonus rounds), but as it is not real-time, you can stop at any time and continue later, as well as re-watch any fight at a choice of speeds.
The interface is stellar. You can make a keybind of your choice to send the entire contents of your inventory to the storage bin, for re-arrangement purposes; you can reserve the items you like but can't afford yet, so that they don't disappear when you pull the rope to re-roll the shop; there's a section listing all the items in the game, and another one for all the item combinations.
All in all, I'll probably be playing this game until I'm Grandma - because that's the highest rank in the game, just above Grandmaster. Backpack Battles is easily my favourite Magic Tetris Shopping Simulator of all time! And it lets me have a giant rainbow blob as a pet!