What about the STONE TREE FORTRESS?
One (1) central column of stone serves as the structural foundation (stem), as well, stone "branches" can be used to go to different parts of the fort (access ONLY from the stem or shared branches on the same branch.). Must be independent from other parts of the fort, only central access allowed (IE-Traffic Hell, unless you have a large enough base to "grow" from).
Stairways are forbidden in this building project, ramps however are allowed/necessary. It MUST be a tree made of stone.
All constructions must start from the stem and not touch ground. Holes in platforms are allowed for defensive measure; seeing as tiles are stone leaves. The branches can be built from "sapling" floorboards, but said floors must be removed or remain until extensions (growth) as additional branches on the same branch are finished; then they MUST be removed.
Hint: if you can build ramps from above, then you may not need to do so much work. But again, construction must be independent from other branches.
Note: You can design the tree however you like, but try to keep it relatively naturally tree-like in form, you can include roots if you like, especially if soil/sand is the foundation. I advise you tunnel/channel and construct from end to beginning. Keep it rather seamless looking.
And please, try to refrain from having tons of hollow zones in the "tree", especially in the base (if it's large). However, if the base is going to be made hollow, make it a proper entrance. And yes, it can even be used as a trade depot zone, provided it's "dense" enough to support the "tree".
Mock those damn Elves with a massive stone tree.
Bonus points if it looks like it's flicking them off.
As well, I previously mentioned this as an
achievement:
 suspended only by stairways, no walls nor supports.&c=1&d=3&e=305&f=1)
Self-explanatory. Just make a pancake fortress. Just be sure it's fully functional. No supports, forts, walls, or ramps. Optionally, you can always use magma as "maple syrup" for the fort, or water to serve as "corn syrup" or even snow or sand as "sugar". Can be constructed from any material. Bonus points for building an adamantium (or high value) fork and knife (5z minimum) alongside. Additional points for roundness of fortress "cakes".
As with both challenges, no floodgate gateway system to prevent access by ground (passive anti-seige defense doors). Same applies with drawbridges (sorry, no loopholes).
Example:
.....
.XXX.
.X>X. <- Upward stairway surrounded by floodgates
.XXX.
.....And one last "plant" related fortress idea. VINE FORTRESS!!!
For best effect, start from the bottom of a really tall cliff and build straight upward and onward with a series of stems climbing all the way to the top. Similar rules to the tree challenge, only ramps, roots are allowed for added effect, you can also make "fruit" bulbs or giant leaves for habitats as you build upward. No stairs, only ramps. Must be constructed. Any material can be used; but rock is preferred for pissing off elves. Can be built in any form, as long as it follows guidelines. Branches can be formed and sometimes merged. And as well, no floodgate guard doors nor drawbridges.
And one final idea. ULTRABEAST SKELETON FORTRESS!!!
Following similar rules (for the foundation structure the fort can be built on/in), semi self-explanatory. Design a fortress around a foundation of a massive skeleton of any creature. The more complex, the better. (Bones don't have stairs, but it doesn't mean you can't make any inside of them.)
Example, a fortress built within, on top of, a giant elephant skeleton. Be sure to use white stone for the bones. As for exo-skeletal creatures (like crabs, spiders, etc.), the bones can be hollow.
Key rule:
All listed challenges must be built independently. Designate a quarry zone away from project area if necessary. If no quarry, then refrain from making a mini-fort of the mining zones, and do not allow the mining to interfere with the challenge (unless roots are being formed from the very stone mined in the walls/floor.)
Additional "piss off the elves" challenge:
Make all "plant"-related projects/forts out of the ash made of surrounding trees. Heavily-forested biomes advised.
Ultimate challenge:
Make any of these out of cast-obsidian.
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