The phrase "a ballroom on" is correct and usable in written English.
It can be used when referring to a specific location or event that takes place in a ballroom, often followed by a description or context.
Example: "The gala will be held in a ballroom on the top floor of the hotel."
Alternatives: "a ballroom at" or "a ballroom in".
His group was playing in a ballroom on the fourth of thirteen floors, on a ship built like a hotel.
Directly ahead is a second rotunda, from which a grand staircase ascends to a ballroom on the second floor.
The rest of the Serfers came later and our first gig was opening for X at a ballroom on Sunset.
The two-day auction will be held in a ballroom on Mrs. Kluge's estate, which is down the road from Thomas Jefferson's Monticello.
The university and the George W. Bush Presidential Library Foundation announced the decision in a ballroom on the campus here, after the university board unanimously approved the agreement.
Not to be outdone, the Hilton Cincinnati Netherlands Plaza has a Lady in Green, who roams a ballroom on the third floor as well as the mezzanine level.
The Music Shed will be transformed into a ballroom for a program on Aug. 19 when the New World Jazz Orchestra plays to an audience at cafe tables.
The students, who have been studying the river and its pollution in biology class at their school, just outside Albany, were among more than 1,100 people crowded into a downtown ballroom on a frigid night for the first public hearing on a plan to clean up the Hudson.
"I have fallen off a motorcycle in a bikini, galloped across the plains on a horse, face-planted on a snowboard, careened into a ballroom wall on roller skates, and landed in a checkerboard of boxes three stories below," says stunt performer Kelly Richardson.
Nearly 400 Iowa ministers and many of their spouses accepted, filling a ballroom here on March 24 and 25.
Then it announced it would hold its own independent "press event" at a ballroom here on Tuesday night.
