Now you can build the artillery guns by using the Special option in the Workshop menu. The artillery piece is a mortar that consecutively fires highly damaging shells with a high level of precision and accuracy. Retrieve the artillery schematics and smoke grenades. Explore the lower tunnels and destroy Sarge. Go to one of your other settlements, Sanctuary works, go to a settler and send them to the Castle, wait about 24 hours for them to get there and. You also need to pick up some artillery smoke grenades to advance the quest.
Once inside, take whatever weapons and gear suits you.
Go back to the main court and open the workshop window after listening to the conversation with Ronnie.