Urban dead followed what was, at the time, a new theme - a city quarantined with a (Romero-ish rules) zombie apocalypse. What was notable at the time is that the game contained no NPCs whatsoever, and only players haunted the streets of the blighted city of Malton.
Death in Urban dead as a survivor turned one into a zombie, and death as a zombie kept one as a zombie (needing to spend AP to stand up). Zombies could be revived by players using syringes (thus turning back into survivors) to give a cyclic game of death and revival. Players who wanted to be revived, perhaps with a sense of irony, congregated in graveyards or outside certain buildings...
The game lore implies heavily that the outbreak was caused by a (shady) ompany named Necrotech - whose buildings were unmarked by players without a certain skill. Like a lot of things in urban dead, however, a lot was left up to the players to decide and so a lot of what gets said is mostly fan-canon.
Core gameplay for survivors revolves around barricading/repairing buildings, setting up (and fuelling) generators, searching for items, and using said items to kill zombies/revive players. Experience was granted for actions undertaken successfully, and player class just adjusted skill costs (with all players able to unlock all skills, eventually). Zombies, on the other hand focused on tearing down barricades, ruining buildings, and hunting down players - their gameplay lending itself well to group coordination to form hoards.
There were a couple of spin off cities to mark zombie-themed cinematic released in 2008; namely Monroeville and Borehamwood whose mechanics differed slightly.
The fan community of Urban Dead were unusually creative, even providing graphics, and creating much written fiction within the wiki, as well as public coordination tools such as mall and necrotech building status pages (upkept by players). Even today it makes for interesting reading, and is a core part of early tile-based MMORPG history.