This is a paid asset, but now you can download Top-Down Scifi modular Environment for FREE, but please remember this package is provided only for learning purposes or to be able to test before buying the product, NOT FOR COMMERCIAL PURPOSES.
Detail this asset from Unity Store: Original Link
Top-Down Scifi modular Environment v1.1
The package is complete modular environment to construct a Top-Down Scifi level for your game.
The set includes modular walls and floors,particles and 2 different high-res texture sets in atlas,PBR materials
Just use unity vertex snapping tool to snap walls and floors and build level you desire
–UPDATE–
– New modular propses to build level
– 12 turrets (some of them have walk animation)
– modular pipes
– Added New .sbsar (substance designer) materials in old and new materials folder, now you can change and have any colors for levels and propses.
(P.S. to use .sbsar materials in unity 2018.1 you need to install a suitable third-party external importer from the Asset store.)
If you dont want to use .sbsar materials there are normal materials in project too