Because it has changed significantly since 2011, we have published a more recent review of Terraria here. We have left this review online as a record of how we felt about it when it released.
Terraria starts like a less satisfying version of Minecraft, squashed into a 2D platformer. It ends as a terrific exploration game, packed with places to discover. Floating islands, festering Corrupted Lands and huge multi-level dungeons guarded by boss monsters are just a few things you can unearth once you've crafted yourself a pair of pants sturdy enough to survive this world.
Building and crafting in Terraria is simple once you know how, but the only help you get upon jumping into your procedurally generated world is a few cryptic hints from a guide NPC. It took persistence and a lot of help from a wiki before I figured it out. Clicking on any block in the world will smash it and deposit it in your inventory, where mined substances can be combined to form increasingly insane items that you can equip, or use to improve your hand-built home. Standing near a furnace or an anvil opens up more crafting options. If you've got the right gear, you'll eventually be able to craft grappling hooks, jet boots and phase swords (lightsabers!).
Now I have a golden broadsword, a ring of health regeneration, metal pants and my very own palace. I flaunt my new copper trousers at the floating eyeball that's attacking me. I've come a long way since my first night, spent madly whirling a pickaxe around my head to fend off waves of zombies.
To find the materials and items to build these things, you're going to have to go travelling, which is where Terraria really shines. Tunnelling into a vast and intimidating cavern for the first time was horrible. How nervously I threw a torch over the edge to find out what new creature was emitting the awful growling noise that had made me tarnish my copper trousers. Then there was the time I found that underground jungle full of nether-hornets, or the giant mushroom forest guarded by a Mother Slime. It feels like there's no end of new areas to discover.
Theoretically you can turn your world into a server and invite as many friends as your connection can handle, but currently the netcode is extremely buggy. One of the regular patches could clean this up, but for now most players resort to using hacks such as Hamachi (a shareware virtual private network) as a workaround, which is not a surefire solution. For the short time I had it working, the multiplayer was more chaotic, faster and more exciting than playing alone.
Combat is haphazard, too. There's little strategy beyond madly waving your weapon at your foes. The warrior with the best sword wins.
If you don't mind this, and are happy to go online for crafting recipes, then Terraria offers dozens and dozens of hours of rewarding exploration. The lack of in-game tutorial and the slow start are drawbacks, but at only £6, Terraria is a steal for those with the patience to reach its deepest caverns.