Carla, You could make this using separate components (molds) and cement them together. There is even a terra cotta cement dye that would mimic the look of this terra cotta clay castle. You'd have to experiment with the dye to get the right percentage of dye to mix. Too much dye and the mix will crumble. Too little dye and you get a strange pinkish look.
Some mold suggestions: some of the terrets look like upsidedown clay flower pots, plastic flower vases, balls, bowls. The bodies can be made with plastic trash cans or plastic jugs. You can also make molds from ridig foam insulation sheets that they sell at home improvement stores. You can cut and carve the foam then duct tape it together.
As each piece is in the first 24 hours of curing, you would have to carve the decorative embellishments. The only thing that might be hard to do is the thumb print shingle look on the two tiered ball terrets. Billie

For more inspiration take a look at this: http://www.roadsideamerica.com/story/2015

The priest who made this place used all sorts of things, for instance the golden dome on a castle is 1/2 of a toilet bowl lift thingy.

Here is a link that might be useful: Ave Maria Grotto