Just designed some modular walls with Blender you can use as prefabs in Unity-3D. I know it is very easy but effective: I use only one material and texture (1024 x 1024) so these walls are perfect for mobile games. You can build a house by modular design or use the wall-pieces to create level borders and city-walls.
The models will be added to the Low Poly Buildings-package, each wall-part or pillar will be available as prefab you can drag & drop into your scene.
The models are designed low poly as you can see here: