A lot of typical challenges wear off after the first year or so. For example, even in the evilest of biomes, if you get dug in and set up some cage traps you'll probably be fine until a deadly dust FB gets to you. The goal here is to have a challenge that continues well into the later years of a fort. The seiges will get bigger and the titans more frequent but the military won't ever be developing beyond sticks and rocks. So I decided I'd like to try this challenge out, add a few caveats (and possibly edits to this post...), and above all make a bit of a plan first before I jump into it.
First and foremost I have a worldgen that I quite like, with ridiculous savagery, zombified biomes, dozens of volcanoes, 500 titans, very few children survive worldgen, and of course there's rather large goblin seiges. It's essentially vanilla DF, even the original tile set which I prefer, but I admit that I have edited almost every animal except domestic ones to be tamable and war trainable (cave crocs are my favorite). I've also used a lot of FPS-gaining-modifications and also some flavor mods, like, all animals create the same bones and leather, all "brown" trees create the same wood, a lot of junk stones and alloys don't even exist, crundles are less common, elephants and rhinos can survive grazing, and GCSs can breed. There is also MUCH less clothing types. That's most of it. It's definitely "almost" pure DF in my opinion.
Ground Rules:
-Use as challenging of a worldgen as you're comfortable with
-Use whatever embark profile you want, but no metal weapons or armor except picks. An anvil is ok
-Embark is 2x2 which will strictly limit your wood supply and invaders start closer to you
-Part of the map must be a reanimating zombie biome, hopefully with zombie Yaks or Harpies or other such challenges
-You may forge picks at any time, and you may use any bars of your choice for artifact moods, but they can't be used by the military
-However, artifact wood, stone, or bone weapons/armor may be used
-Keep a lowish population cap, perhaps 100, so it's not so easy to meat sponge seiges and zombie hordes
Economy:
-The trade depot is not wagon accessible. You will still get "enough" things brought to you like wood and cloth, but it won't be very much.
-No caravans (only traders on foot) means humans and dwarves won't bring their armies in to help defend you
-You can trade anything you want but it won't matter because there won't be very much to buy
-You probably don't want to waste dwarves on crafting stuff anyway when they could be training
-Anytime you buy or "acquire" metal items you should dump them or melt them into bars immediately
-A wooden training axe can cut trees, don't use metal
Metal:
-You can smelt if you want to
-You can even forge things to level your smiths if you want to, but you need to immediately melt the stuff and never use it
-You may only have one anvil and one forge in your fort and it ought to be in a seclusive area deep away from the main fort
-Metal blocks, furniture, chains, etc are not permissible. Melt them.
-You may set up artifact metal furniture in your fort
-Artifact metal weapons and armor must be horded away in a vault, unusable by military
Military Armor:
-Bone greaves, gauntlets, helmets
-Leather armor, helms, high boots
-Leather or wood shields
-Elven wooden armor
-Layered cloth/leather cloaks, hoods, trousers, mittens, and socks (seriously, you want these, every little % helps)
-Leather quivers, backbacks
-Leather or glass flasks
-No mail shirts

Military Weapons:
-Wooden or Bone Crossbows and Wooden or Bone Bolts
-Obsidian Swords
-Wooden training spears, training axes, training swords
-Elven wood weapons
-Wrestling
-Shield Bashing
Military General:
-War animals of any type are fine, mod what you want. The best ones are probably in the caverns.
-Ballista and catapults certainly can be used with only wood and stone
-Feel free to use whatever you can capture; minotaurs, necromancers, werecreatures
-No danger rooms or upright-spike-falling-training. Go train on troglodytes and local wildlife.
Defense Plans:
-There is always a path into your fort and the path does not change or oscillate
-You can use locked doors or hatches but not wall-ins or drawbridges
-You may only use bridges for engineering projects, everything else, like going over a moat, is floors or paved roads
-You may not use atom-smashing bridges or cave-ins to kill the invaders
-You can use channels, moats, z-levels, cliffs, pits, etc
-You can use water
-You can use magma but you'll either need nether cap magma pumps or a magma piston or a volcano
-For both water and magma defenses you need to use doors and floodgates at the trap entrance, not bridges. An unbreakable artifact floodgate would be nice
Traps:
-You can build as many levers or pressure plates as you want and as many hatches, doors, floodgates, grates, etc as you want
-You can only build 1 cage/weapon/stone/spike trap per season. You may not assemble your first trap until Summer.
-Weapons and cages in the traps can't be metal
-Unfortunately, you may only use masterwork core quality mechanisms and weapons and cages in your traps. Start leveling immediately and recycle your cages!
-Obviously if you make it to year 5 then you can have 20 traps around the map, but will you have 20 masterwork mechanisms and cages?
-You can rearrange your traps at any time.
Tools:
-You can use dfhack bugfixes, clean all, clean owned, prospect, reveal, showmood, temperature fixes, FPS fixes, cursecheck, and embark anywhere
-You can only use autodump if something got stuck underwater or inside a fortification or something
-You can't use fastdwarf, siren, liquids, removebadthoughts, or create item
-You can savescum if your having fun
Reward:
-Everytime your fort successfuly creates a true artifact (not simply the military named weapons or armor) you must make an individual and personalized vault, or showcase area, or fortress defense contraption for it
-After you set the artifact in it's proper place and surround it with guard dogs you may approach the anvil
-You are entitled to forge exactly one masterwork metal item of your choice for each true artifact your dwarves have created. Melt anything else.
-You can "save up" the masterwork items that you are entitled to and make them later, but you can never melt, trade away, or redo it.
-Choose wisely.