What are the final preparations for a Dungeon farm before killing Plantera?

Question

I have killed all 3 mechanical bosses, and now I'm farming chlorophyte and killing more to get the best hallowed/pre-Plantera gear (that's what the game is about for me). I've set up an ocean grinder as well. I'm going to set a farm up in the dungeon after I killed Plantera, and I want to build the grinder before I do so.

What preparations do I need to make to make a safe dungeon grinder? I have a huge amount of dart traps, lava and some spear and spiky ball traps I got from the entrance of the Lihzahrd Temple (they generated outside of the door).

How deeply should it be built? I've heard that the deeper you go the more monsters will spawn. Is this true?

Preferable background? There are specific tiles that some monsters need to spawn according to the Wiki. How do the walls affect the monsters?

Answer 1

I'll preface by saying that I have never set up a grinder in the dungeon. I just ran around killing things with battle potions and water candles for a couple hours to get all the gear I needed.

What preparations do I need to make to make a safe dungeon grinder?

I am not actually sure that you can. Quite a few enemies in the dungeon can pass through walls. Several can teleport. There are also many attacks that can pass through walls, including the Paladin's hammer and The Ragged Caster's Lost Soul Bolts. The Ragged Casters in particular can hide just off screen and shoot directly at you.

How deeply should it be built?

I can't find anything that would support the spawn rate being higher at lower levels. You can use water candles and battle potions in the Dungeon, though.

According to the Wiki, the Dungeon has 2 different spawn rates depending on the layer you are in:

+-----------------------+------+-------------+
|         Biome         | Rate | Max Enemies |
+-----------------------+------+-------------+
| Dungeon (dirt layer)  |   97 |          15 |
| Dungeon (stone layer) |   86 |          15 |
+-----------------------+------+-------------+

What that means is that every 60th of a second, there is a 1 in Rate chance of a monster spawning. As you can see, the difference in the Dungeon is pretty negligible. Other factors should probably take priority, but technically building it in the cavern layer is slightly better. You will have to figure out the cavern layer depth outside of the dungeon by taking a GPS and finding where the background changes. You can get a guide from the depths listed on the wiki page:

+----------+-----------------+---------------------+-----------------+
| Map Size | Min Start Depth | Average Start Depth | Max Start Depth |
+----------+-----------------+---------------------+-----------------+
| Small    | 120 ft          | 480 ft              | 840 ft          |
| Medium   | 180 ft          | 720 ft              | 1260 ft         |
| Large    | 240 ft          | 960 ft              | 1680 ft         |
+----------+-----------------+---------------------+-----------------+

For more information, see the page on Monster Spawning.

Preferable background?

That depends on what items you want to get. As you said, some monsters only spawn on specific backgrounds. For those purposes, you could almost consider the dungeon 3 different biomes-- one for each wall type. It might make sense to set up a farm on each background. You might also try to set up your farm on an intersection between the 3 backgrounds so you can get spawns from all of them. I should note that the backgrounds behind the foreground blocks do not necessarily match the adjacent walls. You might be able to get other enemies to spawn in an area by mining away some of the foreground blocks.

I have a huge amount of dart traps, lava and some spear and spiky ball traps I got from the entrance of the Lihzahrd Temple (they generated outside of the door).

The hard mode monsters in the dungeon are actually really powerful. I would actually recommend going to the lihzahrd temple first to get some of the traps there so that you can set this up with traps that will kill monsters in a reasonable amount of time. You can go in there as soon as you kill Plantera. You don't need anything special to get the traps and you won't need to beat the golem if you don't want to.

Answer 2

I will say that Dungeon farming is tricky as I have built a couple different ones. The issue is that 2 of the Post-Plantera casters in the dungeon are painful and can bypass walls with their spells.

However, since you are doing this Pre-Plantera, A simple lava pit with a either a honey pool/campfire/heart lantern (or all 3!) on a floating platform can keep you healing well enough and eliminate the Blazing Wheel threat. Basic Dungeon Farming Pit this is not an AFK because of the chance of Ragged casters and a Diabolist now and again. (I have not searched out the tiled walls I need to remove, and the pygmies sometimes get bunched in the honey pit)

My pygmy setup, that unfortunately uses the staff so it is post-Plantera. Using a summoner group, like pygmies, ravens, or sharks (Duke Fishron staff) can keep the riff-raff cleared out including the Cursed Skulls and Dark Casters. Two water candles and a battle potion and holding a third water candle spawns thing very nicely if using summon groups. I like to put a Safe house (but I stole the statue for something else) out of harms reach with a king or queen statue in it to get a merchant NPC so I can sell stuff. I like to build it below the cavern level, where the Dungeon gets more convoluted.

The really nice loot comes after Plantera though, and that is trickier. I build an arena with just brick walls, and possibly some slab walls if I want sniper gear. The tiled wall creates the Diabloist whose attack is a persistent fireball that does lots of damage and passes through bricks, so I don't like that one. Ideally, the area you build should not have the tiled walls within a full screen from your safe spot to keep the Diabloist from spawning. Traps active, but I forgot my battle potions Notice the side walls around the honey pool. They are because the paladin is deadly if he has line of sight on you, and Bone Lee is just crazy fast and jumps very high, and finally, the sniper can shoot you from off screen if he has line of sight. So basically you need a spot that puts the spawns in harms way before they can get line of sight on you.

EDIT: Just for fun last night, I ran that farm for a little bit, maybe 15 minutes. For notable drops, I got 2 sniper rifles, a scope, a paladin shield and a bunch of miscellaneous but very saleable items. It filled a chest and half my inventory, for just sitting in the honey, and dodging the errant Ragged Caster that showed up about 3 times. So even without the Battle potion, that one works well for me. Since you can't see it well, in the pit I have a piggy bank and a safe, as it makes switching out stuff and storing coins easier.

Answer 3

What you should do is set up a honey/campfire/heartlamp/ healpad and get full tiki armor a raven staff, optic, staff, and a pygmy staff and then get a beetle thing ( i forgot its name but if you have a pygmy staff in your invetory the witch doctor will sell it) and a summoner emblem which increases minion damage and put your minions traps and lava to work. This works for afk. Even better is build a room down in the dungeon where you have your healing post and respawn there.