Works just fine. I use this defense in my current fortress. 30 marksdwarves can make short work of nearly anything. They don't use fortifications or even a twisty path for goblins to wind back and forth on. Just a tight grouping of 30 marksdwarves doing training in the barracks right before the main doors of the fortress. There are no archery ranges, just an artifact armor stand make into a small barracks.
By the time they run out of ammo the goblin onslaught is crippled. They then beat the survivors to death with their crossbows while the pincushioned survivors try to slowly crawl away to escape.
Setting this up is very easy as well. Just make the best armor you can, with the best metal you have access to. Put the ammo stockpile just inside the main gate, close to the barracks the marksdwarves are in. Only make metal bolts. Wooden and bone bolts are garbage. Then put your squads on active/training and just leave them like that. They will teach each other combat skills and automatically engage any hostiles. They will train up very rapidly without needing archery ranges.
At first what they lack in accuracy they will make up for with volume of fire, but you should have elite marksdwarves within a few years from using goblin sieges as target practice. Then you get both accuracy and volume of fire, which is a fearsome combination.
Here is my setup:
http://mkv25.net/dfma/poi-27136-gatehouseI ought to upload an updated version. Same layout, it just looks nicer and no more temporary ramps.
Those 30 marksdwarves will slaughter everything in short order.