Welcome to Armorama buddy! Looks like your off to an excellent start on your first diorama. Im a huge resin fan myself, and I can say, that is ALOT of resin lol. Well one of the first things I noticed was the open space. Thats definatly a big no no. I say, you squeeze in those building together to where it can fit in with the width of the bridge that you have. I would go to a picture frame store pick up a nice wood picture frame, then go to michaels and pick up some thing peices of balsa wood for sidings. I would take the bridge and slip it perfectly into the picture frame, (no extra spaces on the sides) Then I would measure how tall the bridge is so you can put a top part to it. Put the balsa peices (specifically cut to fit perfectly the height of the bridge) on the sides. Do not glue the buildings down before you paint. Outline it on the picture frame bottom. Your going to need ALOT of rubble for this, so if you want, you can sculpt your own bricks or buy some of verlindens, or a couple other companies bricks and rubble. Youll have to have alot though. So an overveiw:
Size down your base (not such open spaces)
Buy picture frame and thin balsa wood
Make the layout on picture frame
Put everything on picture frame (dont put buildings, or bridge, or water fountain until painted.
Glue some cobblestone down (dont paint it)
Glue rubble down (dont paint it)
PM me if you have any other questions.