The original decode here, as confirmed by Q in drops 2711 & 2712 (almost a year later), was that the southern border wall was to be built by the United States Army Corps of Engineers (military), whose logo is a red castle who have a branch location in Greencastle, Indiana.

u/#q2711

I forget what the original decode on the Stage_5:5[y] was. I believe it was in reference to a Twitter proof or something.

Obviously, double meanings exist, so it's possible this is a reference to something else as well, like OP suggests for the red/green castle here.