New Day, New Trouble

The rest of the night passes without major incident. After defeating the oni, the party is able to return to their base for the night. Occasional sounds of skirmish come from the tunnels as the svirfneblin hold off the kobolds through the night. More curious are the sounds rising up from the valley floor. There are shouts from time to time, and the sound of something heavy being moved, but no attack comes.
In the light of morning, you notice that the kobolds and company have been busy in the night. There are three ballista set up lower down the hill. Each of them has a crew of five, with two hobgoblins and three kobolds around each one. There are two groups of attackers between each ballista that charge as the first shots are fired. Each group consists of three hobgoblins, 10 kobolds, and four winged kobolds.
Ballista, AC15, HP 50, can fire at a target once every three rounds. Their range is 120/480, so the assumption is that the kobolds crept pretty close to the fortress atop the hill in order to set up their siege weapon. I think setting up at 150 feet makes sense, with their first shots targeting the building itself. Given the size of the building, the disadvantage from long range attack is negated.
The walls are meant to withstand damage, but most of the outer walls have fallen in the fortress, and the remaining building has been fortified from the rubble as needed. It is not meant to withstand direct assault. The walls have a damage threshold of 10, with 35 hp damage enough to bring the building down, causing 3d6 bludgeoning damage to anyone inside when it collapses.
The two groups will rush forward to engage anyone coming out of the fortress, while the ballista try knocking the building down. The kobolds don't mind blocking the entrance to the dungeons, or losing allies in the process. They are able and confident tunnelers, and the overall plan is to trap the players within and overrun the tunnels to take them.
If the two groups are defeated, the ballista will continue to fire as long as possible, with the hobgoblins rushing forward to engage while the kobolds continuing to fire. If the hobgoblins are killed, the kobolds will abandon the ballista and race back toward their allies below.
The PCs have the option of attacking the ballista directly to break or disable them, or of defeating the defenders and recovering one of the massive firearms for themselves.
Following a defeat, the players can take a short rest to recover before the next assault begins in early afternoon. Recovering the ballista may prove to be a useful option...