Update!
Over the past couple of days I have slowly tweaking my build. The alcoves have been glued to the top of the towers, then magnetic strips added to keep the bridge in place. The bridge has been sanded to fit into the alcoves with magnetic strips added to meet the strips in the towers.
I cut three foam disks, stuck them on top of each pillar and then covered them with modroc.
Cutting into the base, where the towers stand and removing a layer of foam for the towers to sit embedded. I have also added magnetic strips to help keep them in place.
Finally I have started to add a little paint to this, with the bridge and pillars dry I have coated them with a coat of stone grey spray paint.
The paint work has started!