Kutsmob the goblin tripped over a kea bone protruding from the dirt next to the pit; he tumbled forward and was greeted by a mass of hard, pointy bones upon landing. He pushed himself up and looked up at the sky, and realized he had fallen into some kind of dwarven bone hoard, like his people kept at the village. He noted a massive stone pillar that stuck between the dirt walls that wouldn't budge, then turned back and followed a small hallway, only to find another gigantic room full of bones and corpses.
"Ber! Ago!" He shouted repeatedly at the small iron grate that let a bit of light flood in to the room. He could hear the sound of curious footsteps through the ground above. Eventually, all of the trolls brought to siege the fort began sitting around the grate and taking turns spitting through the holes while the other goblins laughed. Kutsmob was confused, and yelled angrily for help, while his allies laughter grew louder. A kitten pranced around the bone hoard without a care while the iron door to the bonehoard was silently locked.
Most of the dwarves below didn't even know about the siege, let alone that their was a goblin sitting in their trash room. The siege itself was massive and a supreme threat to the entire fort, but goblins are stupid and make great entertainment at any rate.