Dungeons have been the core of Minecraft's adventure aspect ever since they were added in Infdev. Notch and Jeb have improved upon the original idea with Strongholds and Nether Strongholds, but modder Chocolatin took it to a whole new level with his Better Dungeons mod. Not only does it improve the existing dungeons, it adds a lot of randomly generated new dungeons with challenges for the player. From pirate ships to pyramids on the surface, through swamp caves and strongholds under it, to vast cities in the Nether, all populated with new mobs and awesome loot. Let's take a look at the structures this mod adds.
Siege castles, lookout towers and similar medieval structures spawn on the surface and provide a whole new challenge in your travels. Though they aren't all completely different and random, the terrain they spawn on provides a different experience with each one. You may find a lonely lookout tower or a sprawling castle. They are populated with armored variations of Minecraft's vanilla mobs - zombies and skeletons. Although the architecture is simple, they don't look ugly and won't ruin your landscape, they do standout so you can't miss one if it's nearby. Like when you discover an NPC village, you will be very excited when you spot one in the distance, skeletons guarding it's walls and iron bars making for scary fences. They aren't very hard, but the loot isn't the best either, which is expected for a structure that spawns approximately twice a biome. They are especially useful for turning into bases if you're exploring far from home.
Pyramids and ancient statues populate the desert. They aren't very big and have the same color scheme (sandstone, sand) as the desert itself but you can easily spot them if you're looking for them. They may not look like much above ground, but underneath lies a dangerous labyrinth of obsidian in which a trapdoor is hidden that leads to a boss monster (no spoilers :D). It's very easy to get lost in the labyrinth and very hard to beat the boss, but rewards will be plentiful after you've triumphed.
Strongholds and swamp caves both generate under the surface, and although they are very different, I grouped them together because of it. Strongholds aren't like their vanilla Minecraft counterpart, and I can't quite explain them properly. The most simple way would be that they are ravine cities. A couple of buildings (some even above ground) will spawn on the edge of a dangerous ravine with lava. These buildings are populated by the same mobs like siege castles, but the sight of a stronghold, especially if you stumble upon it from below, is even more spectacular than spotting a castle. Even though they don't pose much of a danger, they are worth seeking out and clearing out for the loot they hold. Swamp caves are a sight to admire. They spawn as a part of caves in spawn biomes. They are very beautiful rooms filled with flowers, vines and pumpkins, and mobs (including wasps) of course. The walls are often filled with ores and there are chests of loot in them.
Pirate ships spawn in oceans. They are small or medium in size and have mobs and a pirate king aboard. They aren't very challenging or have good loot, but they do look good off the coast so think of them as “free ocean decoration”. Maybe in the future the creator could make them spawn in or near massive pirate coves? I think that would be an awesome addition.
Nether cities, holes and dungeons spawn in the Nether and are quite rare and hard to access and beat. Nether cities are large, incredible cities that spawn in a lake of lava with a lot of different buildings and mobs. Almost every building contains a chest with some sort of good loot, and there are a lot of them, these cities are much bigger than anything else this mod adds or Minecraft villages. Nether holes are holes that bust through the bedrock ceiling and are an awesome sight while the Nether dungeons are just big square dungeons of netherrack with mob spawners and loot inside. Both the cities and the dungeons are quite a challenge because they are usually surrounded by lava which makes it easy to die, but you will be rewarded with exceptional loot if you manage to beat them.
It's also worth noting that the mod does improve Minecraft's default dungeons by making them bigger with more rooms and better loot. This mod is very awesome and you should get it right away, it only requires ModLoader and does not overwrite any classes so it should be compatible with 99% of the mods out there. It adds a lot of challenge to Minecraft, and makes adventuring that much fun. Download it here.