My latest one, is a Goblin Pinball machine, with 6 flippers.
My fortress has two ways in, the main peacetime entrance, and the pinball entrance. If invaders or undead elephants show up, and get close to the fort, I pull a lever that raises the drawbridge door to the main entrance, leaving the pinball entrance as the only other way in.
invaders come down a length of 3-wide hall, to a huge 10-wide hall divided by two pairs of 5 by 10 bridges, with a small island between them. The top of the pinball machine. One of each pair of bridges is alway up, and the other is always down, so there is always a path across. the first one to get to the center island hits one of 6 pressure plates, activating spikes in the first hall. Those spikes arent meant to stop anything, just to add to the blood and confusion. Then, as they get partway across the second pair of bridges, the levers are pulled, reversing the up and down bridges, dropping anything on them into a 6 by 13 pit 5 levels deep. And at the bottom, there are two 6 by 6 bridges for the lower flippers. And, in case any of the guests dont care to go near either of those flippers, there are the BONUS BALLS! On the lever below the top, there is a garbage dump over each flipper, where rocks can be dumped. When the flippers fire, the rocks go flying all over. So the smart gobs that avoid the flippers, get pelted with falling rocks! And, of course, every pinball machine has at least one drain, where the balls get taken out of play. In this case there are two of them, the spaces underneath the flippers. Anything in those areas when the flippers come down, gets taken out of play instantly and permanently!
And, just as a backup in case anything gets past the pinball machine, there is a pretty standard long hall of death, with both regular weapon and cage traps. And! In the event of a flier coming in and bypassing both of those areas, they end up in a roofed-over courtyard, with a fortified bunker next to the main door. Anything coming across that courtyard is fair game for the crossbowdwarves in the bunker. And, as a fall back to that, there is a team of melee troops waiting in the hall inside the door.
As a bonus to all of this, the bridges were made from blocks, and the hallways were smoothed all the way out to the road in front of the walls. So caravans can come and go thru the pinball machine. At least they can if they are allowed to. But if the elves get too uppity, it might become an Elvin Pinball machine!
A lot of good metal and trade goods might be lost in the machine, but the entertainment it provides is well worth it!