Welcome to the forum, timjames....
Your compositing of the 3 images looks good at first glance.....
Altho I'm not certain whether the castle window was s psd scenery with empty view, I don't detect any stray pixels along the sides of the columns.
But I do see remnants of the original background surrounding the "princess" - on the upper arm, elbow and cuff ; a bit on her abdomen, and a big piece between her hair just below the chin. You may want to edit out that big piece. And like there's something coming out of her upper arm?.... unless that's hair that's missing a part..... or some left over from its original background.
You may have to either erase or defringe the princess layer to minimize the strays as well as soften the edges with a bit of blur or feathering. And probably a bit of lightening hair edges on top of head with the dodge tool.
Over-all..... Maybe it's just me..... but you may need to give the viewer's eye something to focus on..... do you want to focus on the princess or the castle background. If it's the princess you'll have to darken the castle scene in the window.... If it's the background, lower the brightness and contrast of the model and add a bit of shadows on her.
.... just my 2 sens.
It's a good job for someone who's just recently tried doing image manips. Keep up the works and don't give up experimenting......