Luring goblin ambushes into my trap zone

So I've been ambushed a couple of times by goblins, and each time I send out squads of dwarves to go and face them (two melee squads, spear and sword, and a single marksdwarf squad) on the open plain.

This results in more deaths than I would like, so I'm thinking of setting up a simple 'trap zone', maybe a hallway 1 tile wide lined with serrated disc traps, with a squad of dwarves at the end to finish off anything that does get through.

But I noticed that ambushers don't really approach the fort, they just sort of stand out there, taking potshots at passers by. Is there any way to lure them in so my traps can chew them up?