It's called "The Unbridled Tower".
Therefore, build an obsidian tower in the centre of the pit.
Agreed. Build the tower from the second lowest level to the second highest (I've heard that there may be some issues with rain should you construct things on the highest level.).
Surround the tower with a much thicker wall that only comes up half as high as the tower above-ground, but goes just as far as it does below the ground.
Kill, maim, and torture as many random sentients as you can manage without making a tantrum spiral, both yours and the traders. Then engrave everything. With any luck you should have a lot of images of madness, insanity, and debauchery.
To go that extra mile, designate one of the seven starting dwarves with one of the following adjectives and one of the following nouns.
adjective: Dark, Dread, Grim, Foul, Evil, Diseased, Cruel, Fiendish, Monstrous, Bad, Baneful, Repulsive, Disgusting, Destruction, Wrong, Vicious, Vile, Calamitous, Depraved, Repugnant, Wrathful, Wicked, Corrupt, Damnable, Hideous, Maleficent, Rancorous, Poison, Obscene, Immoral, Villainous, Sinful, Nefarious, Malignant, Loathsome, Harmful, Beastly, Sinister, Venal, Devilish, Diabolic, Dire, Deadly, Injurious, Ruinous, Venomous, Dwarven.
noun: Lord, Master, Warlord, Dictator, Chief, Chieftain, Boss, Judge, Patriarch, Captain, Conqueror, Controller, General, Overlord, Matriarch, Preceptor, Princep, Father, Mother, Destroyer, Oblivion, End, Void, King.