as the users above say the game do have multiplayer/co-up.
And reason there are not so many opens servers are that people can just jump into a game and take your best stuff and then jump back to their own game with your stuff.
I dont know if the server admin can prevent this somehow but i hope so.
Also some people just come to damage your buildings and ruin your game.
Well basicly you dont want to play with everyone even if you could.
But game do have multiplayer and it make fighting the boss more easy if you work together with friends.