My current fortress, Ceilgmansion, is currently well under construction. It wil consist of a gigantic circular fortress spanning most of the map. (5x7 embark location, and it'll go pretty much end-to-end in the horizontal direction. Did I mention it's entirely built out of blocks?) The entire lower floor is ringed with fortifications to shoot down from. There will be eight access stairwells throughought the map, in each of the eight compass directions, as well as a main staircase in the middle.
The stairs to the second floor (which, incidentally, is the barracks floor, and I have 3 champion squads off-duty nearly full-time... hehehe...) are halfway between the main staircase and the access stairwells. To reach them from the main staircase, enemies will have to traverse a long corridor with two ballistae at the end (and I have 3 legendary siege operators). To reach the up stairs from the access stairwells, they will have to traverse another long corridor, with a large chamber in the middle that does not have fortifications, rather it has a few floor hatches. Two floors above is a water reservoir with hatches. So, the plan is to lock the doors to the chamber, pull the "drown goblins" lever, wait for them to die, close the top hatches and pull the "flush" lever.
Also, I have two 1-marksdwarf pillboxes outside the current entrance. One tile, surrounded by fortifications (and a ceiling above), with a moat around it, stairs coming up from underneath. In the three sieges I've survived so far, they've done a pretty good job of ventilating the enemy squads and making them flee well before my melee champions are needed.
Which is good, because some of my champions are married women. I've lost 4 babies so far. Dwarves are the best parents ever.