Guide:Enemy farming

Monster farming is an essential part of Terraria. Some items have a very low drop rate; the way to get around this problem is to build a system that will constantly kill monsters until the desired item is obtained.

Monster farming basics
The most essential part to monster farming is knowing the basics of NPC spawning. Monsters will always spawn off screen, their spawn rates can be manipulated, and there is a limit to how many monsters can spawn in an area at one time.

Spawn rates can be changed by a number of factors. First, spawn rates are different depending on the biome and the time of day. Also, spawning is different depending on what depth layer you are in (e.g. dirt layer, stone layer). These factors also change what type of monsters spawn, so make sure you know the environment well before you start farming for a specific type of monster.

A few other effects will modify spawning rates. If there is a Blood Moon, the spawn rates will dramatically increase, and monsters will spawn even near your house. Water Candles and Battle Potions also greatly increase spawning rates. Both these items are recommended if you are farming monsters.

Another very important detail is that no monsters will spawn around a House if there are NPCs housed there currently. Note, a Blood Moon will override the no-spawning zone around houses.

Lava Trap
Lava traps can be used to farm a few types of monsters, but usually become outdated as the game progresses. There are however a few types of monsters that farm really well with this type of trap.

Zombie/Demon Eye Farm
This farm uses lava in a pit to kill Zombies and Demon Eyes. Usually the primary items obtained from this trap are shackles, Zombie arms, lenses, and gel when built pre-Hardmode.