The saucer also happens to have a very curious visual feature - a series of squares forming two bigger squares, glowing the intense blue familiar from warp engines across the series. Some have suggested these might be arboretum windows, but why would arboretums have intense blue lighting? Or windows, for that matter? Sternbach's own blueprints mysteriously (suggestively?) leave this area of the saucer undescribed... A crucial tactical system not included in the blueprints in case some Klingon purchased the set?
That's no warp engine, according to Andrew Probert:
Aft saucer module of Enterprise-D
Probert: They were just supposed to be more windows. Probably lounges of some sort, if I recall, but again, what you're seeing are windows that are broken up. I think that there were larger windows initially.