Few ways to imrpove:
The outside is okay, but you see, the grass is too green. Dark Green looks better in lighted maps and light looks better in darker maps, I know this was a quick project, but try to add more detail instead of duplicating the same things over and over. Another thing is, try to make that field bigger its...small. Last but not least, the underground base is pretty empty, add some more design and creativity.
