A few issues:
-The tiles for the base of the pillars don't match up with the other tiles in the room, they also lack shade tiles to the right of them as well. The center walls lack shade tiles as well to their bottom and right sides.
-The garden itself for being such a focal point of the map seems rather small compared to everything else, not to mention it seems outright odd to see an outdoor section surrounded on all sides by an indoor area. Most castle gardens I've seen in some sort of medieval-fantasy setting are are vast and open, not to mention they have man-made walkways that run through them. We can't expect the high and mighty nobles to walk on bare earth, now can we?
-To parrot deranger's comment the map as a whole seems pretty generic and has too little going on. When designing a map you need too account for each area and make sure that most if not all sections serve some purpose, whether it's a pathway to get you from Point A to Point B, a pathway/area to funnel enemy reinforcements to impede your progress/give you stuff to kill so you can level up and watch your stats go higher/make your playing experience hell, a side room that has treasure, etc. Check out the tutorial that was posted and work from there.
Edit: Okay, so you reworked the map and it does look a little more lively, but I feel like there is too little going on for the topmost part of the map You could cut that part out entirely and still have a serviceable map, and not a bunch of empty space that doesn't contribute at all. Of course that is assuming that you start at the southwest stairwell, even if you start at the north there will be one side on the left or right that doesn't serve much purpouse. Of course, there are ways of fixing that...