Working in a new world right now... I'm going to build as many things as possible, taking as long as possible, because I need to test the stability of my mod over a long period of time.
The mod I'm working on causes water erosion in Minetest... So when exposed to water, stone has a chance of disintegrating into gravel, and gravel into sand. Also when exposed to water, cobble has a chance of becoming mossycobble.
Anyway, I built this interesting tower first... thought I'd post a few screenshots. I am using a custom texture pack.

Here's the whole tower. You can see the erosion mod at work, because some of the cobble has become mossy.

Whole tower. Different angle.

Looking in through the front entrance.

Looking up the central shaft of the tower.

That tiny little floor holds two furnaces.

Looking out the window at the top of the tower, I realize I have a rat problem...