Clearly Goblin sieges fit the first though less typically used definition of a siege, being a direct assault on a fortification.
Once mobile siege engines come into play and Goblins figure out how to tunnel, we have our serious threats.
Hopefully this is after civilian dwarves quit running into fire, military dwarves quit taking their children to battle, and all dwarves realize that safety equals good.
Then, there will be a need for Goblins to give it their all.
Along with the downward aimed battering ram a-la the kraken caller of Pirates of the Carribean, there are many creative siege engines that would develop naturally from war with dwarves.
My two cents on mobile siege engines... wagons seem to move just fine. Siege engines were basically built onto their own wagon. Cant fire while moving, but you can load, then move, then fire.
I can see it now, the goblin forces spawn in at the edge of the map, troops and siege operators and a couple trolls. They all begin a charge. As soon as the catapults are in range they stop, lock wheels, and open fire. Trolls in front formation, ready to bust doors, spring traps, and beat in walls they don't like. The troops, whatever kind of soldier they may be, sheathe weapons and beginning digging into wherever their spades can hit. Their lasher commander screaming in mocking tones, "Strike the earth! Strike the earth!". Dwarven units are firing back with their catapults and skirmishing the goblins, avoiding the trolls as best they can to let traps and champions do their job.
Meanwhile, goblin ambushers are also elsewhere on the map, unspotted as of yet and digging furiously as well. They'll be spotted just from getting near dwarves, but that doesn't change the mission. Make a safe hole for dwarf death.
This siege is typical, still defended against, at the loss of troops, the loss of nearly all the siege engines, the springing of many traps, the loss of walls, doors, and the addition of entrances not meant to be. If these things are not fixed before the next siege, they may all pour in...
"BARON! Shut up about the damn bronze crown! We're in a damn war here!"
...oh, and the noble oblivious to the tantrum spiral he may start at any moment.