I loved this game, and was fortunate enough to play it at its height. But I can also see the reasons for its decline.
The free browser zombie game Urban Dead was an example of player-driven gameplay. With no NPCs, humans and zombies could organically fight for territory, set up groups and plan battles with real consequences. It was a unique and fun experience.
It was also a horrifically and inherently unbalanced game that managed to give both sides gigantic advantages, in likely unforeseen ways. Individual humans could do far more than individual zombies. Zombies essentially cannot communicate in game at all, and it’s far easier for a human to build barricades than a zombie to destroy them. In individual play, a human can do a lot more.
However, groups of zombies are more or less unstoppable. Because they can just stand up after being killed, in a weird “DETERMINATION”-style system that preceded Undertale by a decade, the only method of actually beating them was to outlast the willpower of the players controlling them. And zombie metagamers turned the in-game communications weakness into a strength, setting up out of game networks.
Because of the PVP nature of the game, any balance changes were bitterly contested, making the community an often unpleasant place. This, combined with the inherent limitations of the game, made the playerbase drop.
There are other factors, most notably the game being incredibly beginner-unfriendly. But its balance was, in my opinion, the biggest reason.
Now, it’s exacerbated. A human can hide in a heavily barricaded building and be safe in normal play (too bad there’s little to do), to a ridiculous extent. Yet with even malls being virtually empty, a small organized group of zombies can attack with basically no resistance. Like its namesake, the game is reduced to shambling on.
But it was fun in its heyday. I remember playing it when I was younger, finding it through (what else) Spacebattles.