I would get some foam and build my own. You can carve all the stone work into the foam with a pencil. Rounded towers can be made by glueing a few pieces together and taking a bastard file or rasp to the pieces after they have dried and shaping them to the disired form and then carve your stone work in.
You could also use plaster or sheet rock and carve the stone work into the plaster.
The paper can be removed from the sheet rock by wetting it and peeling it off and then you can carve your stone work in.
Round towers can be made by pouring plaster into small buckets or large cups and stone work caved in.
With a little bit of work and a good imagination you can make something real nice and have fun doing it and come up with a one of a kind piece that fits your needs and can be custom made to fit and compliment your pieces.
You can make draw bridges and other little details from balsa wood or plastic hobby sheets and add thatched roofs from dried grasses. Just use your imagination and let it run wild. The skies the limit. Have fun!!!
How does that monty python holy grail scene go about they thought I was crazy to build a castle on a swamp???
The first one feel over and sank, the second feel over caught fire and sank
the 3rd ect, ect but the 5th one.
Bottom line don,t give up, give it a try and each project will be a learning experience and each one will get better and you,ll have fun doing it.
If you have any kids they,ll also enjoy doing a project like this with you also and if they get in your way just give them something easy to do or just give them a few scraps to do there own little project and they,ll have a ball helping you or doing there own little project.