What image editing software are you using?
If you want to keep the rest of the picture, you just need to cut the castle and move it to the new position, then fill in the sky area.
Actually doing this can be quite complicated depending on what program you're using. In Photoshop you can select the castle with a very accurate outline, move and resize it and then clone the sky over the blank area.
I'm reinstalling Photoshop after a reformat, so I'll have a go at fixing your image over the weekend if you can post a bigger version of the picture.
If you want just the castle without the rest of the picture, cut the castle and paste it over a sky background. As the viewpoint will be looking up at the castle from below, you could maybe put some small clouds around the bottom of it or something like the image below.
