How were you able to get it to cut the curve of your arch, then? (Sorry, I haven't actually used Constructor...using 3DSMax.) What if you were to just rotate that? It looks like it'd work great for a tower.
On the inside shots, the stairs need to be finished. At the least, put some kind of connecting support rails beneath the steps. (Yes, it's fantasy, but it should still look believable.)
Your walls appear to be too thin for the architectural style. I would (at least) double their thickness. Your wooden products appear to be spot-on, though.
I assume the first picture is an inside-the-walls view....a door on the outside wouldn't work at all.
With only the one wall showing, and no other doors on the 3rd pic, it's hard to tell if this is a corner or mid-wall tower. (just ignore this paragraph...It's late!)One thing I would consider adding, though, is murder holes, both in the tower, and the wall itself, especially on the sides, and along the top of the arch.
I know it's not a finished product, and I am not trying to belittle your work. I just have a nasty habit of spewing out the first things that come to my mind as I look at things for the first time. Initial impressions and all...after all, it's what the customer is ultimately going to judge us on!
Scale at the top of the wall and tower doesn't look right on the first pic, after taking another look at it......