AFAIK there are no restrictions on making a similar game to dungeon keeper or any game. The copyright laws only say that you cannot use any original ideas or artwork from the game. You could even market it as "like dungeon keeper" on the box. You just have to be careful about using any of their artworks or original ideas.
Some concepts, however, are so general as to be non copyright. For example, if you make a game based upon a player building a dungeon that is completely allowed, even if your game play is very close. Just be careful not to use any original terms used by the game.
Now personally I think you shouldnt use an existing game idea anyway, even if it is legal. You should ask yourself why you like certain games, and then try to make an original game based upon what you like about them.
I mean it would look other than the original for sure. But the
game-idea would show the roots!
That is totally legal. As a matter of fact legally you could even name it something like "dungeon protector" or something very close, especially given the generality of the words: 'dungeon' and 'keeper'.
Say for example if I wanted to make a doom clone, as many exist already. I could call it "destruction" and make it occur on a mars station. I could send the player through a portal of hell. All of that stuff is completely legal. On my advertising box I could even say: "Just like doom 3" There is no law against that.
Some people might think that you are just a copycat, but I think that if you really love an old game and you want to ressurect it, I think you are giving a lot of respect to the game and it can be cool.
But if you copy just because you have no ideas it is probably a little tacky.
(That post was much longer than I expected to write)
edit:The reason why I know a little about this is because I wanted to make a hardwar clone, or an elite clone a few years ago. SO I checked a lot of resources.
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