You could simply take a cube object, blow it up, and slap a house texture on it. Just browse through google images for 'free building texture' and import that assest into the engine, drag the texture to the cube, and it will be applied. Other than that, you'll have ot build your own building in your main modelling program from scratch, probably starting with basic primitive geometry.

What I do is use a cube, and then use a texture so it looks like a building. This is because I am using scenes with thousands of buildings. You may want to actually make your own in a 3D modelling program, such as blender. Also, you can get some models for free -- check out turbosquid.com, and search 'building'.

This is my first time, making a castle in Unity. So, I have a lot of questions. Ummm, does this apply to creating a bridge as well? And where can I find textures and assets for the castle? Just google image?

1. Create a terrain in the scene if no terrain yet.
2. Use square brush to raise the terrain to the height you want it to be.
3. Use paint texture and select appropriate texture to paint the building.