If you only look at Terraria for a second, especially when you first start it, you may have the idea that it is very similar to the games of yesteryear. No wonder - its pixelated graphics in a grid world, is is basically designed to look like a classic game.
Things start to get interesting when you progress in the game and you start to see the game regularly throws dozens to hundreds of moving enemies at you which can also produce projectiles (pirate invasion... oh how many times I die). The whole wiring system that is in there to activate machines and teleporters is also massively complex and requires a big chunk of CPU time when you start to really use it everywhere.
Then there is the water flowing system (which is getting a major visual improvement in the next patch) which historically tanked the performance on even very powerful PCs if you'd make an entire lake of lava move. Its quite optimized now, but it was a challenge for the developers.
I know the Amiga had some pretty nifty hardware with which you could do things where the PC required quite a bit more CPU horsepower, but I'd be mighty impressed to see an Amiga Terraria with the same feature set.