At its heart, Terraria is a sandbox mining and crafting game. The tags on GOG are certainly not what I would have considered giving it, but then I suppose 'mining and crafting' is a little too niche to be a tag.

You start off in a randomly-generated world with no armour and rubbish weapons. You need to craft new ones and build a shelter before night falls, else you'll be shredded by zombies. You're given free reign to venture out and explore, being mindful of new zones that appear with new and stronger monsters/enemies.

At its heart is the crafting system. You can mine up various ores to use to craft new weapons and armour. It is what I've seen people call a 'wiki game' which means that either you're willing to spend the patience checking with the guide what various things can be made with what, or keep a wiki guide nearby to check up on things.

I've found it fun to play alone, though you do have to have a willingness for repetitive tasks (mainly mining for ores). There's staggered progression in it so that most of the downtime between bosses is usually spent ensuring you have the best armour available.

It can take a few hours to really get into -- I bounced off it when I first bought it, finding it difficult and obtuse, but I dived back in a couple of months later with a wiki at hand. I'm at the point where I've got the three main bosses defeated and am working on defeating a fourth optional boss while I wait to take on the last main boss before the second (and much more difficult) half of the game.