Just use Unity's RenderTexture. Take a look at the example in the manual (near the bottom of the page).
You can quickly build on that process by using a Quad (GameObject -> 3D Object -> Quad) instead of a cube.
- Create a new RenderTexture asset
- Set the resolution to 64x64
- Set AA to none
- Set Filter Mode to Point
- Assign the RenderTexture to the "Target Texture" field of the Main Camera
- Create a Quad (GameObject -> 3D Object -> Quad)
- Turn off Cast & Receive shadows on the Mesh Renderer
- Drag the RenderTexture asset onto the Quad to use the RT as its material
- Change the shader on the Quad to use "Unlit/Texture"
- Create a new Camera
- Set this to Orthographic
- Set Size to 0.5 (assuming that the Quad is of unit size)
- Align the Camera to the Quad
- Create a new Layer
- Assign that new layer to the Quad
- Set the new camera to only render that layer
- Set the Main Camera to exclude rendering that layer
Tada! You got yourself a low res 64x64 setup in Unity. =D
Essentially the "Main Camera" captures all the gameplay, and the other camera will just render the Quad and nothing else. The only problem now is building a player with a 1:1 aspect resolution. I have not found an answer to that yet other than what nothke has said about Screen.SetResolution.