Here's where I am now with it:
For the construction, in the back of my mind I had the Fighting Fantasy illustration of Yaztromo's Tower as inspiration, that slightly asymmetrical, jutting tower with several floors leading up to some sort of magical observatory.
And thus this is how the build started with a plastic drinks bottle for the main structure. However I soon realised that the plastic was too flimsy and filled it with some expanding foam which had been left lying around by one of the builders. I then superglued/epoxied a few more interesting plastic packaging shapes to the exterior (including some sort of fairy light) and raised it up higher with a piece of thick cardboard tubing and some sort of water filter container.
I then decided that I wanted to make a scenic base for it, partly for decorative purposes but also because it was very top-heavy and it needed to be stuck down on a larger stand, in this case an mdf off-cut. Some blue expanded polystyrene scraps were used to build and shape the base, a Hobbycraft skull from Halloween cut and added at an angle and then all the surrounding areas built up further with that same expanding foam (which itself was then cut and shaped to look more like rocks). Finally some cocktail sticks were poked through and a sprinkle of sand for small rubble.
Some details were needed and it just so happened that I already had a stash of resin doors and windows (I can't recall where from) and these were super-glued into place. The last stage was the addition of lots of polystyrene bricks, cut up from some unwanted furniture packaging. These were cut into roughly equal sized brick and stuck on with a glue-gun over several evenings. They were then textured by rolling a ball of silver foil over the surface (this could have been done first for ease in hindsight - or I've even seen people put them all in a bucket with a few stones and smash them around). Finally, once attached, I covered all of the polystyrene in several coats of glue to stop the primer from eating into it.
The next steps I need to address are to add some twisted wire to emulate vines creeping up the side and to add some sort of wizardy gargoyle design from my bitzbox to fill that empty plinth in the middle. Perhaps some sort of dragon or flying creature? I'm also toying with the idea of carefully suspending a ping pong ball above the parapets at the top or to make a telescope? Thoughts on this please would be much appreciated?