I wanted some interesting engravings, so I decided to give my dwarves something to watch in the hope that they would carve it. I built my fortress with a 40 tile long entrance hall. I then channeled the floor and put retracting bridges over it. I did the same on the next lower level. And on the level under that, I put some upright spikes linked to a repeater. On the side of my "holding chamber" and "blending chamber", I put fortifications, and windows behind those fortifications, and an observation hall behind those windows.
So, over the course of several years, whenever a siege or ambush squad would show up, I would drop them into the holding chamber. The benefit of a holding chamber is that the siegers don't retreat since technically, nobody is dead, yet. I built up a sizable army of one or two hundred goblins in my holding chamber. Again, after a number of years, I decreed it ready. I took a handful of dogs, and pastured them in the entrance hall. I drafted my engravers and stationed them in the observation chamber, and then locked the door. I then pulled the lever, dropping the dogs into the mix of goblins. The goblins tore them apart almost instantly. Anyway, while this was not a nice thing to do, I wouldn't consider it particularly horrible; however, I neglected to mention one rather crucial detail. I had embarked in a reanimating biome. The dogs came back to life. 4 zombies vs. 200 goblins? They died pretty quick again. However, the dogs would not stay dead. Until one of them got lucky, and managed to tear an arm off one of the gobs. Slowly, but surely, the chewed and ripped off limbs overcame the hundreds of goblins. It was difficult to see what was going on through all of the miasma. All I know is that the number of goblins dropped. Slowly at first, but more and more quickly.
Once all that was left was a bunch of zombie limbs and corpses, I allowed my engravers to leave while I prepared the next stage. I dropped the zombies to the bottom level, and turned on the automated spikes, thinking that eventually the pieces would start to rot away. (Incidentally, they didn't. I just got more pieces.)
So, I started gathering sieges again. After a few more years, I had hundreds of goblins. I like to think that they could hear the sounds coming from the chamber below. I once again stationed my engravers, this time in the lower observation chamber. I then dropped the new goblins in to pit of repeating pikes and zombie limbs. It was over very quickly.
So, after years of preparation, I considered my engravers ready. They had honed their art smoothing the entire fortress. They had watched hundreds of goblins getting torn to pieces by the undead. The images surely must be etched upon their psyches. I had them start engraving the trade depot first. Nothing but circles, dwarves travelling, a titan moving into a cave, etc.