Hi Green-Pilgrim. That's a great start. I have grappled with some of the same questions you raised here, so I'll refer to my own maps you can find by clicking on my sig.

1. Roofs are the trickiest. I think your roof is fine the way it is - I don't advise making any changes. The three towers over the main keep have grey tiles, as opposed to the red tiles elsewhere. Maybe change the colour to a slate blue. The best tower roof I ever did was Enc 14 of Post 1 of the Madness at Gardmore Abbey series (see sig.) but that style won't suit you I think, however the colour will.

2. Walkways - yup this is always tricky. My most successful paths on grass are Encounter Maps 19 & 20 on Post 1 of the Madness at Gardmore Abbey series I did. You can see some less successful attempts on Encs 9-12. An example of a subtle well-used path through a castle courtyard is post 77 of Castle Jacktannery - It's a low-opacity layer of diffuse mud texture overlaying the paving stones done with a layer mask. Obviously my examples are not applicable because we have different styles, however you can see from them what colour combinations work and don't work, which might help you. The second question should not be a major issue for you because you are not using a photo-realistic style so whether a path is gravel or mud is irrelevant; for you what matters is the colour. I would use the same military-style pattern of the grass.

3. A drop shadow won't work (assuming you are using an automated computer version that offsets a layer of shadow a set amount of pixels to the left and down) in this case. You need to do it by hand - it is easy (but I do not know which programme you use so I'll be vague)

a) -Make the grey colour of the lower part of the tower roof very slightly darker (say,10-15% less brightness - use a slider) than the uppermost part. If you had three different levels you would use three shades of grey, etc.
b) make a new shadow layer. Draw a very thick black line along the step between the two elevations, and then blur it a LOT. Keep blurring this line until it is completely blurred and you can barely make out that's it still there. It has to be very subtle. If you overdo it you can always duplicate it. If you use the hand blur tool it will look more natural and better than the quicker 'blur layer' tool, but you are aiming for such a gentle and difuse blur that it won't make much difference. This shadow layer must be at least twice as blurry and subtle to suit your style than I would need to do to suit my own style, which is much more forgiving.
c) crucial last step - remove all the parts of the shadow layer EXCEPT the part over the lower elevation of the tower roof. You can not do this by hand because you have to be absolutely accurate it it will look wrong. There are various ways to achieve this - best perhaps is to go back to layer A) and select that dark grey colour than invert the selection and delete it from the shadow layer.

That should work. You may need to experiment slightly with the amount of blurring - unless it is EXTREMELY subtle it will make the rest of your map look bad, since it will be the only shadow on your map. It must be VERY subtle.