In my last two fortress's I've rigged up long atom-smasher corridors at my entrance just for Goblins and other nasty invaders, Size 10 beasties excluded of course. But its all in the name of warfare, cause I know eventually the Goblin Sieges are just going to get ridiculously bad. Hell, the last Siege at Nitigrigoth had to have had around 100 Goblins at least considering I caught over 60 of them in cage traps alone and I STILL got at least 8 that breached my defenses through a weak point and a large squad ran off after I killed the leader who was IN that 8.
But I set it up like this. W=Wall, F=Floodgate, B=Bridge.
WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWW
WBBBBBBBBWBBBBBBBBBWBBBBBBBBBWBBBBBBBBBWBBBBBBBBBW
FBBBBBBBBBF BBBBBBBBBF BBBBBBBBBFBBBBBBBBBF BBBBBBBBB F
WBBBBBBBBWBBBBBBBBBWBBBBBBBBBWBBBBBBBBBWBBBBBBBBBW
WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWW
Now, while the uniformity here is off. There is absolutly no space for any invaders to move or hide in. And I have the control levers inside down in the main area so if I order my Dwarves indoors I can operate the floodgates and the bridges. So, the enemies HAVE to funnel in through the atomsmasher to even approach my fortress. And the road is a tight squeeze designed to limit their ability escape. Bassicly, in case of crap, lock em in, make Goblin pudding. Bill Cosby tested, Bill Cosby approved.