This place is amazing. It is in a classic building truly overlooking Grand Place. It has been modernized (it even has air conditioning--unique to my Europe travels) without losing the charm of the original architecture.
It is a walk-up with narrow, twisty stairs. We traveled to Brussels with only backpacks but if I'd had actual luggage, the climb might have been awkward. But even the stairs and hallways, which seem original to the building, are charming.
The Grand Place is the center of tourist activity in Brussels. Great restaurants, jazz clubs, and other cultural venues are within a couple minutes' walk. It doesn't get any more convenient.
That said, the apartment overlooks the square and, due to the single-pane glass (historic preservation probably prevents them from replacing them), you can hear the crowd below. However, we didn't need to use the provided earplugs in order to sleep.
This is definitely one of my favorite Airbnb stays!