Using blocks and NPCs from other mods
The Great Wall Mod and the Walled City Generator can use blocks from other mods in custom templates. Special effort has been made to adapt mob spawners so you can spawn modded creates inside cities and walls. I've included with the SSP downloads templates using NPCs from Humans+ (Humans+ compatibility currently inoperative) and Castle Defenders. See here for instructions on using these templates. See the documentation for more info on modded blocks.
This world generation mod creates ruins from cellular automata. CARuins can take the form of towers, bizarre statues, pyramids, mysterious elevated temples, or huge multifloor underground dungeons. They can stretch deep underground and are populated with hidden chambers and loot. Since they are generated from random seeds, there is a near-infinite variety of form, and no two CARuins you encounter will be alike. The 2D cellular automata used (including the famous Game of Life) have been selected from among the thousands possible in order to produce interesting structures.
Walled City Generator
Creates large procedurally generated cities in your world for you to explore. Cities are based on a customizable format that lets you customize many aspects such as materials, size, density, and rarity to your desire. Cities also include custom wall styles, buildings, and dungeons to conquer. Currently eight city styles come with the mod, including friendly cities inhabited by villagers, several "ruined" city styles filled with hostile mobs, cities in giant underground caverns, and a city that spawns only in the nether. SSP and vanilla SMP versions are available.
A recent review showing Underground Cities thanks to Etalyx:
The Great Wall Mod
Generates walls, road, and buildings. By default it creates terrain-spanning structures resembling The Great Wall of China. Roads can be created by using ground-level walls. Walls are generated at random during map creation and head off in a given direction while following the contours of the terrain. They will continue on until they reach an obstacle they cannot surmount, often running for many hundreds of blocks. SSP and vanilla SMP versions are available. Preview video thanks to MCmodShowcase
By generating roads and buildings at high density, the Great Wall Mod can create inifinite cities. Try using the Casbah.tml template along with GlobalFrequency 0.5 or higher in the GreatWallModSettings.txt file. Alternately any wall template should work if you give it a short min_length. Climb a tower to see the view . More info here.
i installed your mod, and the first world i generated had a wall made of blocks that had a moving water texture on all sides, and broke like dirt, but after that world, none of the worlds i make have walls on them.