Scattering 3 wide wood platforms through the tiers of your layers will allow ground base/air based units to move in between the layers of your farm without you having to hop up and down the entire length of the farming chamber. Once you have completed your layers, you require a reset point roughly 40 blocks from the case of your mob farm.
This will be your workspace, everything inside here will contain and focus the mob spawns into you and make killing and collecting their goods easier then farming them in the wild. Its height and width are unimportant however the best design is 80x80 (roughly 15 layers with a 4 space gap in between them.) In this case you make layers of whatever material is best used to create the biome you desire.
'''Jungle - Mud with jungle seeds
'''Hallow - Pearlsand'''
'''Corruption - Ebonsand'''
'''Corruption - Ebonstone'''
'''Hallow - Pearlstone'''
The layers inside the case are constructed out of the biome material and the case is created out of Bricks or ores.Each layer needs a 3 wide wooden platform every 20 blocks for walking/flying creatures to drop through to the collection area down below. The bottom layer should comprise of an extended passage roughly 50 blocks in length in which to reset the spawns within the chamber and some form of medium which will split flying and walking mobs. In this example I use wooden platforms across a pool of water for the collection point. The flying mobs are restricted to above the water and the walking mobs drop through.
Glowing mushroom seed spread will compete with jungle seed spread so make sure you create a standoff between the mud patches of 3 blocks.
The hallow can be seeded using pearlstone of hallow seeds into any underground reason you choose to place it. The units spawned however will vary depending on the height from the surface of the world. If you place it below the rock layer you will spawn in the harder variants of the hallow creatures which make soul of light farming slightly more difficult.
To complete this case just follow the original design with several tweaks, one will be the layer in which you build your case. Another will be ensuring your case is completely surrounded by pearlstone to avoid corruption creatures spawning inside your farm. Refer to Zone Restriction for more on this. You will need to ensure that for your reset position you create a wall off with a door otherwise you will get spammed down by skeleton archers and soak unnecessary damage every time you reset the farm.
'Mobs:'(dependant on how close you are to the surface)
'Surface Hallow(above rock layer)'
'Deep Hallow(Below Rock layer)'
The corruption is one of the easiest biomes to seed as it will manipulate the environment it is seeded in for best spreading(i.e changing mud into dirt) for this reason it cannot be colocated with any biome beside Dark Desert and should be sited as such. This case will also need internal safety area's(walls) with doors installed to avoid taking unnecessary damage from clingers while you are farming. This case can be built anywhere as the units that spawn inside it will not change regardless of positioning.
Ensure that you encase your corruption biome in a brick type or some other non-corruptible material to avoid having your biome spread out of control. You will need to create a doorway for your reset site to avoid the multiple ranged attacks offered by the corruption biome.
One of the easiest to maintain and best paying out, adding two layers of sand to the top of your corruption/hallow case will cause the sand to becoming corrupted/holy and start spawning mummies of the varied biome types at a higher rate then normal deserts spawn them as you will have a much higher surface area across the length of your case then most natural deserts.
To create a normal desert you must ensure the case is sealed by brick. This biome is extremely susceptible to manipulation by the other two biomes. In addition you will need the case to be at least 3 deep to ensure no spread to your artificial desert.
Some mobs will spawn regardless of the biome you have created, dependant on the layer you have created your mob farm on. These common spawns can be something of a nuisance but nothing can be done to control them short of building on the surface and ensuring you don't build high enough to pull aggro from the wyvrn
However as a plus side the large surface area's of the cases have a positive side effect of increased spawn rates of mimics.
This is mostly unnecessary but I include it to make something obvious. Hell is rich. Creating a bridge of a good length over the lava and farming the Wall of Flesh is the best way to make ridiculous amounts of money there is. No case involved, just summon wall, kill, wash repeat with a payout of 10 gold minimum plus whatever he drops(most of the items he drops sells for at least 5 gold minus the pwnhammer.) Creating a case for hell is something of a waste of time because of the teleporting critters.
Pwnhammer- 78 silver
Clockwork Rifle- 3 gold
Breaker blade- 3 gold
Melee/Sorceror/Range Emblem- 2 gold
''''''This has to be split into two sections: ''''''
''''''One, methods for keeping your varied biomes clean and;''''''
''''''Two, methods for increasing spawn rate in your farms by zone restriction.''''''''
First thing you need to know is that you require a minimum of a 3 block buffed to guarantee biome spread does not occur. The materials for your buffer have to be artificial. Brick types or placing varied ores but it must not be made out of stone, dirt, mud or sand or your buffer will fail and Corruption/Hallow will spread from or into your case. An additional method for segregating your case from surrounding factors is by cutting a 4x4 hollow around the case leaving a gap which should halt spread. This method is less effective as certain biomes(Jungle, Hallow,Corruption) spawn plantlife from dirt blocks that can and will spread into/out of your case if you are not careful.
Now, this is the most important part of zone restriction and it is what makes farming like this successful, Spawn rate boosting. This works on the premise that only a certain number of creatures will spawn in the world at any given time and said creatures use YOU as their spawn point, They will only spawn off your screen, however they are spawning specifically because you are there. If you build your case without denying spawning in the area around the case the mobs will be spawning in the surrounding area as well thus dropping the spawn rate inside the case itself.
Thus, you need to make all the area around your case uninhabitable by mobs. This is done through a systematic denial of zone. No mob in the game can spawn in a 1 width or 1 height area. Thus, you must dig out every open space in the vicinity of your case and then fill it with a block in lines. You can completely fill the area but this is both time consuming and resource heavy, splitting the area into single lines which leave half of it open air and half closed off disallow spawning while at the same time granting you vision of the area which you have sealed which make burrowing creatures easier to deal with. Keep in mind the more zone denial you do the better the results you will have inside your case(more mobs spawning).