Zombie Siege is a server built around one thing: surviving endless waves of zombies. It's a custom game mode made from scratch, with exclusive plugins that give it its own feel instead of the usual copy-paste setup.
As you push further in, you can make use of personal vaults, progression ranks, a full shop system, and item repair to keep yourself going. There are also multiple maps to play on, so there's a bit of variety while you try to hold out against the horde.
If you're into PvE with a survival twist and a more custom, adventure-style setup, Zombie Siege is made for that kind of grind.