The navmesh settings used in part four of the tutorial Creating Enemy #1 is inaccurate. It was defined to work with Unity 4.x and doesn't work properly in Unity 5.x
This forum post from someone from Unity describes the correct settings that should be used for Unity 5.x.

Commands/shortcuts

Duplicate Object = CTRL + D
Opens reference to documentation = CTRL + '
New Empty GameObject: CTRL + SHIFT + N
Write "inf" in the inspector textboxes transforms the value to Infinity

Projects Settings

Setup Github for Unity Projects: link
Gitignore for Unity: link
Check Infos: Edit > Project Settings > Input

NavMesh to define the walkable areas in the levels.
Event Systems to detect and handle user input and scripted interaction.
Animator state machine to control and play all of the character animations; including idle, walking and interaction.
Prefab system to save the character so it can be easily added and used in any scene in the game.

Scripting

Event functions / Default Mono Behaviour:

Initialization Events

Awake = The Awake function is called for each object in the scene at the time when the scene loads.
(it is called regardless if the script is enabled or not), all the Awakes will have finished before the first Start is called. This means that code in a Start function can make use of other initializations previously carried out in the Awake phase.
Start = The Start function is called before the first frame or physics update on an object.

Regular Update Events

Update = Update is called before the frame is rendered and also before animations are calculated.
FixedUpdate = FixedUpdate is called just before each physics update. It is important to note that the physics updates and frame updates do not occur with the same frequency.
LateUpdate = Runs at a point after the Update and FixedUpdate functions have been called for all objects in the scene and after all animations have been calculated.

Time and Framerate Management

Time.timeScale = Time Scale property that controls how fast game time proceeds relative to real time. It can produce effects like: Bullet time, slow motion or even stop the game.

void Pause() {
  Time.timeScale = 0;
}

void Resume() {
  Time.timeScale = 1;
}   

Creating and Destroying GameObjects

Instantiate = Create/make a copy of an object (the original object doesn't have to be in the scene).

GameObject enemy;
Instantiate(enemy);

Destroy = will destroy an object after the frame update has finished or optionally after a short time delay.

Destroy(enemy);
Destroy(enemy, .5f);//destroys it after short interval
Destroy(this);//destroy the current script and not the GameObject itself!

Attributes

Attributes are markers that can be placed above a class, property or function in a script to indicate special behaviour.
[System.Serializable] = add to class to make public fields visible in the editor.
[SerializeField] = add to private fields to show them in the editor/inspector.
[HideInInspector] = prevent the property being shown in the inspector, even if it is public.
[Range(float,float)] = displays a slider in the inspector using the defined values.
[Tooltip(string)] = provides information in the inspector about the related field/property.

UI

Unity UI is based on Canvas.
UI Elements have Rect Transform instead of Transform.
It is good practice to add a canvas group to the main Canvas. Canvas group allows UI elements to have an alpha and it allows you to toggle whether they are interactables.

RENDER MODE = The render mode can be set to Screen Space - Overlay so it will fit the screen; Screen Space - Camera which can have UI perspective and World Space, which is for things that are intrinsically within the 3D scene. So for example you might have a speech bubble popping up from a 3D character.

Objects/Components Behaviours

#rigidbody
-->MovePosition

#Vector3
-->normalized

.setActive(bool); // GameObject
.enabled = bool; // component

Unity's coding good practices

Public variables always before private ones.
Use namespaces to separation of concerns.

To add later:

prefabs brief description;
use of forward instead of addForce;
Input.GetAxisRaw() != Input.GetAxis(): There is difference in the return values, check what it is;

Add a Light Component to the player and enemy ships but make it disabled, enable it during shots so the shots can have a nice lightning effect.

Add a player healthbar that also displays the shield value. Right now the player is destroyed if touched by an enemy/asteroid without a shield.

Add a powerup similar to the shield that already in game. This new one should allow the player to shoot multiple shots from his spaceship.

The animation configuration is correct in the Animator window, the transitions exist and are configured as they should. The code in the PlayerMovement script is also correct, still the player doesn't play the walking animation.

Created a trello board to work and organize projects ideas and the status of tasks.
trello board

Every Text that is presented on the screen has its own canvas. The HUD is disorganized and it is hard to work and add new features.
It is necessary to create a single canvas element, add a group and organize all the Texts inside it. Also, update the references in scripts to this new approach.

SpaceShooter in-game Image:

In the Space Shooter tutorial about score it is used the GUIText component. It is obsolete and the recomended one now is Text (from Canvas).
It is necessary to modify the code to make it work.