You found a [Dungeon][1]. These contain [Monster Spawners][2].

The `spawn-monsters` [multiplayer option][3] in the server's config file only controls *natural*, random monster spawns in dark areas. Unless the server difficulty is also set to Peaceful (or 0), Monster Spawners will still be active if you go near them.


  [1]: http://www.minecraftwiki.net/wiki/Dungeon
  [2]: http://www.minecraftwiki.net/wiki/Monster_Spawner
  [3]: http://www.minecraftwiki.net/wiki/Server.properties#Minecraft_server_properties