Are game developers allowed to copy each others game design?

It seems like whenever I go onto the Iphone app store there's 50 more games that came out that are exactly like castle crashers although they changed a "few things" Is the legal? Could Supercell possibly sue these copy cats for stealing their design of game? Or is it completely normal to have competition like this?

They could try suing. But it'll take them 50 years to sue every company.plus it depends if castle crashers uses original concepts or gameplay that didn't exist before this game. And whether they have patents to protect their intellectual property

It's fine as long as the recreations didn't digitally rip any thing from the original game. (If they recreated all the tiles, sprites, textures, music, sound, layout, logic, it's perfectly fine.) I'm guessing that these recreations came up with completely different looks though.