As of 1996, after the events on the Hunter's Moon, it is also the headquarters of the Gargoyle Taskforce, headed by Matt Bluestone, advised by Margot Yale. Aside from the GTF, it presumably sees regular Manhattan police work. ("Invitation Only")
Michael Reaves is likely the writer to have given the police precinct its number.  Greg Weisman has written that there was an effort to portray the work at the 23rd Precinct realistically. For example, Elisa Maza's investigations might take her beyond the precinct's boundaries, but she only began investigating crimes that occured within those boundaries. 
Real World Background
Coincidentally, while the show was on the air, the actual 23rd Precinct building in New York City had (as recently as 2007) a facade of a castle.  Currently, the building now houses the Traffic Control Division for the NYPD.