I found these invitations that open in the center a few years ago on a close out sale at a stationary store. I decided to finally use them to make Lydia's princess party invitations. I had pre-folded invitation paper, but there is no reason that you couldn't make something similar using cardstock.
I thought the center opening would lend itself well to a castle design. I found a graphic that I liked and then spent some time formatting and getting everything to print correctly. I started by creating a word document the size of my invitation paper BEFORE folding the paper.
Insert your castle into the document and size it to the size the card will be after folding. The basic idea is to take the one image, split it in two (I made a copy then I cropped both in half) then move the half pictures to the paper edges so that the cut hall of the castle is on each edge. The biggest issue was that my printer didn't want to print all the way to the edge of the paper. I tricked it by sizing the document a little longer than my paper really was. The beginning edge still had a small border (margins as narrow as possible) but the last edge to print had the image going off the edge. The paper overlaps a little when the invitation is folded, so the blank border is hidden behind the printed edge. That created its own problem in that the picture now overlapped so my perfect half castles didn't line up perfectly anymore. I just went back to the crop tool and moved the cropped edge over a little more. It wasn't exact, but it seemed faster than doing all the measuring involved to figure out how far to move the edge to compensate for the overlap. I just printed and folded and checked my work and made adjustments a couple of times to get it right.
That's it. Simply print your details in the center of the other side of the invitation or print them on a separate card and insert it into the center. Either way, you open the castle to see the information about the party.