Ok, I'll keep this short and sweet. I masked the spire and then copied it to a seperate layer. I cheated a bit and used Vertus' masking filter to do this - it simplifies matters but the old-fashioned way of masking will work just as well, if not longer.

From there I cloned the background over the original spire, just in case I wanted to keep the background.

Then I duplicated the spire layer four times, resized each and made them look they followed the horizon towards the center and back.

I then duplicated-mirrored these four and placed them on the other side.

I then warped them so that they curved around the center slightly to give me arches. Spherize might have worked as well, liquify would do too (for older Photoshop versions)

Then I placed the part of the second image that I wanted to use.

Then came the hard part - I mirrored the dome to make it more symmetrical, blended the middle in a few places that didn't align well with the smudge tool at about 20 pressure, and then began my warping.

Not really sure why I warped the parts I did, but I know that for some reason I kept thinking of an old 19th century Spanish attack vessel design... anyhoo, again, older Photoshop users would need to use Liquify to achieve this result, and if wanted it to still be symmetrical, would have to just copy/paste one half over to the other side.

Finally, to clean it all up I added small effects. I used faded find edges to give me a sharper, more defined image. I used Lucisart to tweak the look of smaller details a bit more, though this could have been done with a burn tool and a LOT of time. I then went and removed some of the saturation of the bricks, adjusted the hues to more bluish tones, adjusted the curves to make an "S" to make lights lighter and darks darker, and then then did a little bit of burning here and there to add shadows. Finally, I made layers of some of the parts that I wanted to make appear "overhang" to imitate more depth, and added very small drop shadows to that purpose.

I may have left out a few small things, like a few erasures or something. I did a LOT of toying with things during this, but I'm pretty sure I hit the biggies so that someone else might see what I did.