Super glue the remaining square dowels to the base, and if you wish to the other dowels to make up the rampart floor. Allow to dry.
Finally, add in texture paste. Use it to cover up any gaps between the rampart and the base itself. Throw it on there as thick as you like but keep in mind not to go too high if you're going to use a basing mix on it.
As you can see this process is very scalable and if you have an airbrush and stay focused you could probably build an entire fort in an afternoon and have it painted by the end of the next day, if not sooner. The materials are incredibly cheap compared to going and buying resin or plastic kits to make the same thing.
Now its done and assembled. Ready for paint if you are using varnish instead of thinned PVA for a sealer. I'll make a tutorial for how I paint them as I paint this one up. Probably tomorrow.