I think it's just way the developer dumps existing functions to a metatable, like I've seen something like "var.print = print" even though until now i still don't understand what's the purpose.
And i think since castle.xyz is engine on top of engineβwhich is love2dβ it could be that it's a love2d function like "love.graphics" whereas it's common to use it for sketching early prototype and testing grids, like "love.graphics.rectangle("fill"or"line",x,y,width,height)",
and since that is in metatable, means there is a barriers, which mean only people who see the block code in the actual castle source code will know how it works and how to operate it.
But this is still hypothetical, and I'm not really sure.
Meanwhile I'm still learning how to make game in Love2D.