The first thing you can assess is the width of the octagonal roof. From the photo alone you can see the bases of the roofs are narrower than the towers, and are also erected over another base which you may have missed.


Also, the octagonal roofs are clearly steeper in the photo, while the pixel art rendition is some 50-60% of the real roof's height.


Arches are always a pain to create in this angle, especially for those who are just getting started. I used to utilise a "skewed ellipse" approach, which involves skewing a ellipse vertically in MS Paint by 26 degrees.


While the result may need more or less trail-and-error amendments, one with enough experience can device a good looking arch though this method.

You might also like to look into creating a consistent flow of pixels on the base of the window frame. Doing so will make it appear less rough.


Yours is a good start for a first timer. All in all, attention to detail is important if one intended to accurately replicate buildings in this form. So keep practicing, focus on minute details, and experiment. Good luck!

From the photo, it seems to me that the base shape of the tower has six sides and not eight. (That is of course completely irrelevant for your pixel art, unless you're planning to create a realistic replication of that cathedral.)