Arena
First off, I assume you've skimmed the Arena guide on this wiki. Here I'll combine the tips rather than separate them by encounter.
Introduction
What
A Terraria arena can be as simple as a bunch of wood platforms, to a complex structure studded with traps and other mechanisms. Personally I prefer simplicity, as many arenas I've seen from other players tend to be overcomplicated and require a lot of preparation and materials. On the other hand, simpler structures leave the emphasis on player skill - so if you're looking for cunning arrangements of traps and teleporters and abuse of the game engine you won't find them here. I'm not saying abuse is bad; things like hoiks have resulted in some very clever arenas. But if those are ever fixed in a later game update, you'll be screwed (remember the 6-trigger statue trick? yeah). Finally, I don't find AFK gaming fun - no strategy, just download what someone else made and let the game play itself.
Why
The saying "Preparation is half the battle" holds true in Terraria. An arena will often mean the difference between a clean fight versus a messy running battle that can easily turn into a rout and failure, not to mention a waste of any consumables you may have used.
Where
The majority of events and boss fights occur around your spawn point. As such, when considering where to build a general-purpose arena I strongly suggest that you build it around there. You may want to build a separate arena for certain other encounters such as against Plantera (since luring it out of the Underground Jungle is a bad idea) or the Wall of Flesh (as it only appears in The Underworld).
When
The earlier you start the better, although I concede there's little need for an arena when you're first starting out. Preferably you should have a pickaxe able to remove dirt and stone with ease, because nothing's more boring than more time wasted on mere digging. Ideally you should have access to the Mechanic NPC as she sells some very useful stuff, although it's certainly possible to fight the pre-hardmode encounters with a basic no-frills arena.
Design
My emphasis is on mobility - what doesn't hit you won't hurt you. This allows squishier characters to play too, i.e. the less armored ranged and mage types. I'm also going to assume that you're building around your initial spawn point at the center of the world's surface.
Terrain
The immediate screen around your spawn point should be flat, extending to about half a screen on either side. Beyond that, dig straight downwards to create a cliff - you will later fill in the bottom of the cliffs on either side with a layer of lava. This is to force enemies to spawn as close to your arena screen as possible. This isn't important for boss fights, but rather for events. If you're worried about where to place your surface farms, just move them further - from the center point, about 2/3rds of a screen's width of land, followed by the cliff then approximately a screen's width of lava terminated by a wall + quarantine gap. Build anything you like beyond the gap.
Ground level
Ideally the spot where you spawn would have 2 NPC houses right next to it: one for the Nurse and the other for the Dryad. The nurse serves as an emergency heal (make sure she is within click interaction range when you appear at your spawn point), and she will keep herself and the dryad alive. The dryad's buff will permeate the lower half of the arena - every little bit helps. Note that their houses should be as narrow and short as possible - the roofs should be level with the floor of your arena. For now you need to jump out of your spawn point since it'll be a hole in the ground, later on you can wire a teleporter activated via player-only pressure plate (this allows instant teleporting merely by jumping on it).
Side note: only these 2 NPCs should be so near the arena. Make sure to bury everyone else deeper underground away from possible encounters. My own base is roughly 2 screens below, accessible only via teleporter. Before I had teleporters I used a tunnel shaft to get down there - I filled it in later to prevent enemies from spawning in it.
It may be possible to take advantage of the new NPC defense mechanic to incorporate them into an arena (so that the NPCs appear to join you in the fighting). The problem is there's no easy way to gather them all into one area (unless King/Queen statue abuse?), not to mention they're pretty fragile. They'd have to be in a position to shoot at the enemies but safely away from being attacked. They're of little use against normal enemies since you shouldn't require any help taking care of those, but they'd die too quickly against bosses (any attack that travels through blocks will basically spell doom for the NPCs). So until the NPCs get beefed up you should keep them away from your arena except for the nurse and dryad - they're simply too fragile to participate in boss fights.
Platforms
The main body of your arena will be several rows of wood platforms about 1 screen's width, roughly 20 blocks apart vertically. Yes, you can't normally jump that high - this is to force you to use your grappling hook. The grapple pulls you faster than simply jumping does, and mastering the technique of slinging yourself around the various levels of the arena is important. Make sure to practice hauling yourself up your arena while you are also running, because in a fight you can't afford to stand still and wait for your grapple to hit something before it pulls you. 5 levels of platforms should be enough although in practice I don't often lose sight of the ground - however, there will be occasions where you need to rapidly increase your elevation to get out of the way of something. It's better to have the extra height in case you need it, plus wood platforms are cheap and easy to craft.
Perimeter walls
Finally, build a vertical border wall 1 block wide between each level of the platforms - these walls should NOT be at the very end of the platforms, instead build them about 5 blocks in. Make sure to leave a gap 4 blocks high between the bottom end of the wall and the platform below it. Unlike a traditional enclosed box, the walls here will still provide some protection against attacks that can't penetrate blocks AND will also allow you to slip "outside" the arena in case the boss is attacking from the inside and you need to climb up. That's the reason for the gaps below each wall (to allow you to run in to/out from the arena), and why the walls are several blocks inside rather than capping the edges of the platforms (to allow you some standing room outside of the arena). See, in a traditional enclosed box-shaped arena when the boss starts lazoring you you're gonna be screwed. In my arena, you can always duck in or out of it on either side of every floor.
This basic arena should suffice for all pre-hardmode encounters. Before you trigger hardmode though it's a good idea to get some basic upgrades in place.
-
- This is a work in progress.