After a few days away I was able to make some early morning progress on the hill. I have used Das air drying clay to strengthen the lower slopes and to fill in the trenches. You can't see much because the clay is white:
I then cut a piece of hardboard to the shape of the fortification; using newspaper to make the initial template:
...and have begun to build up the earth walks using more clay. This was a rapidly constructed fortification rather than one with lots of reinforcing timber so it will look a little "rough & ready".
I hope to build up the walls further tomorrow. These will eventually be covered with sand to add texture before painting.
The fortification is removable allowing me to use it elsewhere , to use alternative fortification on the hill or to cover the trenches with pieces of grass matting to use the hill in an unfortified state.