Lunar surface? Alright. Firstly, I know you’ve got experience in art, so first rule of thumb.
NEVER. EVER. MAKE. A. NATURAL. FLAT. SURFACE.
So many beginners make flat surfaces for a natural landscape. Make highs and lows if you’re making a natural landscape, because otherwise it’s…well…not natural.
Currently, it looks basic and way too flat, then again, you’re asking for what you need. Put a rough, scratched sort of surface (Kind of like dusty roads/rocks). Then make some hills on a few grid squares, and/or depressions, to give the land some texture.
Add decorations/land features and you’re pretty much done.
Also, only one side? Maybe put a huge hill behind/something because aliens only attacking from one side = dumb.
Btw, the barricades are obviously all the same. Maybe a randomised texture for barricades, selecting from several types or just pure randomisation?