NPCS

Q: What are NPCs?
A: NPCs are characters such as the Guide, who appear to help you in various ways. There are seven NPCs in the current build of the game, and they will appear once you have an empty house and have met the individual requirements for each NPC.

Q: How do I build a house for the NPCs?
A: Houses need to fulfill a set of requirements to be considered a proper house. The step-by-step guide to making a house is as follows:

-Build the walls and roof. The room must be five tiles tall at the least, not counting the roof itself. Additionally, the room must not share a wall with any other house. You can circumvent this by building two-tile-thick walls seperating each house. Make sure the room is big enough to be considered a proper house as well.
-Fill in the house with background walls. Wooden walls are recommended, but stone walls work too. Make sure everything's covered up.
-Place a light source inside the room. A torch on the wall works fine, but you can also use candles and other such fancy items. You can make said fancy decorations with metals at an anvil.
-Place a Wooden Table and at least one Wooden Chair in the house.
-Place a Door in the house, in one of the walls.

Assuming you've done everything right, the game should treat your room as a house. The first house you build will be taken by the Guide, and any further houses by whichever NPCs appear. Seven houses in total are enough in the current build.

Q: Who are the NPCs, and how do I obtain them?
A: The following NPCs are avaliable in the current build:

-The Guide, who is present at the beginning of the game and provides advice when right-clicked. He'll take the first house you build.
-The Merchant, who buys and sells items for money. Once you have at least fifty Silver Coins, he will move into an empty house. The Merchant sells Anvils, and the very useful Mining Helmet.
-The Nurse will appear once you've found at least one Life Crystal and upgraded your maximum health. Right-clicking her will allow you to fully heal your wounds at the cost of roughly 2 silver for 400 hp (max).
-The Dyrad appears once you've defeated any boss. She sells Purification Powder and sunflowers for combating the Corruption.
-The Arms Dealer appears when you have found a firearm, and sells ammunition and lower-tier guns.
-The Demolitions Man (aka the demoman) appears when you have Dynamite. He sells Grenades, Bombs, and Dynamite.
-The Clothier (aka the Old Man) appears once Skeletron is defeated, and sells Vanity items.

Q: NPCs aren't moving in, even though I have houses and have met the requirements!
A: Ensure your houses are properly built. Additionally, make sure that the NPCs can actually reach the house's door. Finally, try waiting a bit. If all else fails, build a new house elsewhere and see if it works.

Q: How do NPCs work in multiplayer?
A: The requirements to attract NPCs are shared across all players. As long as one player has a gun, for example, the Arms Dealer will come. The money requirement for the Merchant is divided amongst all players; all the coins in the inventories of all players is combined, and the Merchant is summoned if the tally is at least 50 silver.