Spikes look like a strip of very sharp metal teeth and are located in the Armaments section of inventory. And ouch! Spikes deal damage when you touch them. Equipping an exo skin will give you immunity, though.
Spikes occupy a 1x1 block and have full mob collision, partial player collision, partial projectile collision, full rain collision, and no liquid collision. In other words, they will stop mobs, will impede but not necessarily stop players, will impede but not necessarily stop bullets or other forms of gunfire, will stop rain, and will not stop or contain water or other liquids. They will also block mob line-of-sight.
They are the only game item that can be rotated and placed in any of the four directions (up, down, left, or right).
How to Obtain
Spikes can be crafted, traded for, or mined from naturally-generated dungeons in all biome types. (Although spikes generate in caves in brain biomes, these spikes are actually just taking the place of stalagmites and stalactites, and when mined give only scrap metal.)
How to Use
Spikes were originally intended to add danger and difficulty to dungeons and player-made challenges. If your goal is to create a challenging build for other players to enjoy, or just a really annoying one, you can use spikes strategically, such as in places where players must jump, or at the bottom of an inconveniently-located acid pool of death. Or if you enjoy finding creative ways to kill yourself in-game, you can build yourself into a tiny spike-studded room and savor a slow death by a thousand cuts.
To change the direction of the spikes when you place them, simply move in the direction you want them to point. They will place according to whatever your last movement was.
Due to their complex collision features, spikes are unique among all items in that when placed facing each other (for example, one pointing up and one pointing down), they allow the easy passage of weapon fire (and even, sometimes, players) while still preventing mob movement and blocking the player from being seen by mobs. For this reason, they're the perfect item for creating an opening into a mob farm through which the player can shoot without risking being fired upon in return.
Since spikes cause damage to players who don't have exo skins, players sometimes use them in unprotected PvP environments to kill opponents by burying them in spikes. They're sometimes also used the same way in non-PvP worlds. This is a nasty way to use spikes, and even though this won't cause karma, it will put a black mark on your soul for the rest of your life.
The collision area of spikes differs depending on what build you're using. In v2 builds, the area is smaller, limited to roughly the visual size of the item, while in v3 builds, it's the size of a full block. This means in v2, it's possible for a player to squeeze between closely-set inward-pointing spikes, but in v3 (the only version available to Windows users), it's not. Since some dungeons were designed on Mac or iOS platforms using v2, there are certain spike-studded dungeons that remain impossible for Windows and other v3 users to raid.