It may be flat but the design is good.

It looks so flat, I could use it as an ironing board. Manupix is right, you must choose a light source to attain depth. Also, this art could benefit by making it less cluttered; make it simple!

There's no shading at all: choose a light source and direction and consider its effect on the houses (go outside on a sunny day and look at the sunlight on buildings!). Some sides should be lit frontally, some laterally and some not; some parts should cast shadows on others and on the ground.

There is an overabundance and evenness of detail which makes it almost boring. Old houses are visually interesting because of their long history of small modifications, tear and wear, neglected or broken parts. Also you don't have to show every single brick or tile: try to suggest instead of describing.