A lot of time can be saved while building hills or castle walls if you use a very thick corrugated cardboard
A typical cardboard is about 3 mm wide (1/8") for a single-wall sheet. This is perfect for most projects. However it is unnecessarily hard to construct flat pieces of terrain like castle walls or hills as these need to be about 30 mm wide to look good and support miniatures (unless you are going for a 2.5D look but this is another universe on its own).
I have discovered recently that you can get sheets of wider cardboard of 16 mm (5/8") or even 30 mm (1 3/16") by looting Ikea packaging. This may be a new thing for them at the time of writing this as I had never seen such sheets before and Ikea packaging has been my main source of cardboard for fifteen years now.
So I say goodbye to polystyrene foams (like Styrofoam) which are naughty and messy to work with. Now let's make another hill.