I get the feeling Wizzy might be sealed under there, thanks to his loss to NiGHTS in the first game.
I agree as well. It might be some kind of reminding for old gamers, like: "Do not forget about Wizeman, he is still somewhere around there, even if he's not visible or sensible" (what I'm saying is that, well, such character as Wizeman could not dissapear once and for all)
Yes, that is also looks like Wizeman/Reala fusion And Sairus made this fusion idea to look even better.
And if it's nothing like that... well, nice gate design then ^^'
Maybe as each level is completed, the other Nightmaren break the seal bit-by-bit, and the last level is when he's loose and out for blood...The crystals might be what's sealing him in...
Or maybe I'm overthinking it and it's a Wizzy cameo.