End spike

Obsidian Pillars (also known as Obsidian Spikes) are generated structures in The End that are made of Obsidian. Obsidian Pillars are not destroyed by the Ender dragon, as they consist of Obsidian. Rather, the Dragon will simply pass through them. They are quite large, with the smallest being 2x2 blocks with a height of 20 blocks. Larger pillars can be 7x7 with a height of 64 blocks, making them suitable for hollowing out as buildings (though the Ender Dragon can still pass through them and injure the player).

Obsidian Pillars do offer some protection in The End, as they can prevent Endermen from sneaking up behind you, and the Ender Dragon generally attempts to avoid flying through them. Fighting from the top of a pillar does prevent Endermen from attacking you, but the Ender Dragon's knockback effect means you are better off fighting from the ground.

There are also square obsidian pillars like this:

Usage
A single Ender Crystal spawns on top of each pillar, which heals the Ender Dragon. When these Ender Crystals are destroyed a Bedrock block will replace the Ender Crystal. Note that the blast radius of an Ender Crystal is always larger than the radius of the pillar, so there is no completely safe way to destroy a crystal while standing on top of the pillar, however, sneaking while on the pillar and destroying the crystal will at least protect you from being blown off the pillar, and wearing a full suit of diamond armor will minimize the explosion damage. If you destroy an Ender Crystal while the Enderdragon is healing from it, the Enderdragon takes damage. This is a quick way to kill the Enderdragon. To get on top of a pillar you can build a ladder, which is dangerous because the Ender dragon could destroy the ladder. This, luckily, is rather rare, because the Ender dragon tries to avoid the pillars. A ladder also comes in handy after destruction of the Ender Crystals, because you can jump off the pillar and then slow your fall with the ladder. Another option is to dig your way up. This is not recommended, as mining obsidian is time consuming. The intended method, however, is to kill nearby Endermen and use their Ender Pearls to teleport to the top of the pillars. As long as one is playing on Easy or higher, and has a nice amount of Food, armor, and potions, the battle will be a piece of cake. And it might not hurt to bring a bow with some arrows to attack long ranged.