So, I have a Problem: The Player can Jump off Walls and Skip Huge Parts of the Level like this.

But this is not intended and I don't want this.

This is the Player Character Controller:

 using UnityEngine;
 using UnityEngine.Events;
 
 public class CharacterController2D : MonoBehaviour
 {
     [SerializeField] private float m_JumpForce = 400f;                            // Amount of force added when the player jumps.
     //[Range(0, 1)] [SerializeField] private float m_CrouchSpeed = .36f;            // Amount of maxSpeed applied to crouching movement. 1 = 100%
     [Range(0, .3f)] [SerializeField] private float m_MovementSmoothing = .05f;    // How much to smooth out the movement
     [SerializeField] private bool m_AirControl = false;                            // Whether or not a player can steer while jumping;
     [SerializeField] private LayerMask m_WhatIsGround;                            // A mask determining what is ground to the character
     [SerializeField] private Transform m_GroundCheck;                            // A position marking where to check if the player is grounded.
     [SerializeField] private Transform m_GroundCheck2;                          // A 2nd position marking where to check if the player is grounded.
     [SerializeField] private Transform m_GroundCheck3;                          // A 3nd position marking where to check if the player is grounded.
     [SerializeField] private Transform m_CeilingCheck;                            // A position marking where to check for ceilings
     [SerializeField] private Collider2D m_CrouchDisableCollider;                // A collider that will be disabled when crouching
 
     const float k_GroundedRadius = .2f; // Radius of the overlap circle to determine if grounded
     private bool m_Grounded;            // Whether or not the player is grounded.
     const float k_CeilingRadius = .2f; // Radius of the overlap circle to determine if the player can stand up
     private Rigidbody2D m_Rigidbody2D;
     private bool m_FacingRight = true;  // For determining which way the player is currently facing.
     private Vector3 m_Velocity = Vector3.zero;
 
     [Header("Events")]
     [Space]
 
     public UnityEvent OnLandEvent;
 
     [System.Serializable]
     public class BoolEvent : UnityEvent<bool> { }
 
     public BoolEvent OnCrouchEvent;
     //private bool m_wasCrouching = false;
 
     private void Awake()
     {
         m_Rigidbody2D = GetComponent<Rigidbody2D>();
 
         if (OnLandEvent == null)
             OnLandEvent = new UnityEvent();
 
         /*if (OnCrouchEvent == null)
             OnCrouchEvent = new BoolEvent();*/
     }
 
     private void FixedUpdate()
     {
         bool wasGrounded = m_Grounded;
         m_Grounded = false;
 
         // The player is grounded if a circlecast to the groundcheck position hits anything designated as ground
         // This can be done using layers instead but Sample Assets will not overwrite your project settings.
         Collider2D[] colliders = Physics2D.OverlapCircleAll(m_GroundCheck.position, k_GroundedRadius, m_WhatIsGround);
         for (int i = 0; i < colliders.Length; i++)
         {
             if (colliders[i].gameObject != gameObject)
             {
                 m_Grounded = true;
                 if (!wasGrounded)
                     OnLandEvent.Invoke();
             }
         }
 
         //The same Part of the Code again, but with the 2nd Ground Check:
         colliders = Physics2D.OverlapCircleAll(m_GroundCheck2.position, k_GroundedRadius, m_WhatIsGround);
         for (int i = 0; i < colliders.Length; i++)
         {
             if (colliders[i].gameObject != gameObject)
             {
                 m_Grounded = true;
                 if (!wasGrounded)
                     OnLandEvent.Invoke();
             }
         }
     }
 
 
     public void Move(float move, /*bool crouch,*/ bool jump)
     {
         /* // If crouching, check to see if the character can stand up
         if (!crouch)
         {
             // If the character has a ceiling preventing them from standing up, keep them crouching
             if (Physics2D.OverlapCircle(m_CeilingCheck.position, k_CeilingRadius, m_WhatIsGround))
             {
                 crouch = true;
             }
         }*/
 
         //only control the player if grounded or airControl is turned on
         if (m_Grounded || m_AirControl)
         {
 
             /* // If crouching
             if (crouch)
             {
                 if (!m_wasCrouching)
                 {
                     m_wasCrouching = true;
                     OnCrouchEvent.Invoke(true);
                 }
 
                 // Reduce the speed by the crouchSpeed multiplier
                 move *= m_CrouchSpeed;
 
                 // Disable one of the colliders when crouching
                 if (m_CrouchDisableCollider != null)
                     m_CrouchDisableCollider.enabled = false;
             } else
             {
                 // Enable the collider when not crouching
                 if (m_CrouchDisableCollider != null)
                     m_CrouchDisableCollider.enabled = true;
 
                 if (m_wasCrouching)
                 {
                     m_wasCrouching = false;
                     OnCrouchEvent.Invoke(false);
                 }
             } */
 
             // Move the character by finding the target velocity
             Vector3 targetVelocity = new Vector2(move * 10f, m_Rigidbody2D.velocity.y);
             // And then smoothing it out and applying it to the character
             m_Rigidbody2D.velocity = Vector3.SmoothDamp(m_Rigidbody2D.velocity, targetVelocity, ref m_Velocity, m_MovementSmoothing);
 
             // If the input is moving the player right and the player is facing left...
             if (move > 0 && !m_FacingRight)
             {
                 // ... flip the player.
                 Flip();
             }
             // Otherwise if the input is moving the player left and the player is facing right...
             else if (move < 0 && m_FacingRight)
             {
                 // ... flip the player.
                 Flip();
             }
         }
         // If the player should jump...
         if (m_Grounded && jump)
         {
             // Add a vertical force to the player.
             m_Grounded = false;
             m_Rigidbody2D.AddForce(new Vector2(0f, m_JumpForce));
         }
     }
 
 
     private void Flip()
     {
         // Switch the way the player is labelled as facing.
         m_FacingRight = !m_FacingRight;
 
         // Multiply the player's x local scale by -1.
         Vector3 theScale = transform.localScale;
         theScale.x *= -1;
         transform.localScale = theScale;
     }
 }

This is the Player Movement Script:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class PlayerMovementNew : MonoBehaviour {
 
     public CharacterController2D controller;
 
     //public Animator animator;
 
     public float runSpeed = 40f;
 
     float horizontalInput = 0f;
     bool jump = false;
     //bool crouch = false;
 
     public float playerDirection; //-1 is left, 1 is right
 
     // Update is called once per frame
     void Update ()
     {
         horizontalInput = Input.GetAxisRaw ("Horizontal") * runSpeed;
 
         if ( Input.GetButtonDown("Jump")) {
             jump = true;
         }
 
         /*if (Input.GetButtonDown ("Crouch"))
         {
             crouch = true;
         } else if (Input.GetButtonUp ("Crouch"))
         {
                     crouch = false;
         }*/
 
         //Get Direction:
         if (Input.GetAxisRaw ("Horizontal") < -0.1) {
             playerDirection = -1; //Left
             //animator.SetBool ("isWalking", true); //Set Animation to Walk
         } 
         else if (Input.GetAxisRaw ("Horizontal") > 0.1) {
             playerDirection = 1; //Right
             //animator.SetBool ("isWalking", true); //Set Animation to Walk
         } 
         else if (Input.GetAxisRaw ("Horizontal") == 0) {
             //animator.SetBool ("isWalking", true); //Set Animation to Idle
         }
 
         /*if (jump == true) {
             //animator.SetBool ("isJumping", true); //Set Animation to Jump
         } else if (jump == false) {
             //animator.SetBool ("isJumping", false); //Set Animation to Idle
         }*/
     }
 
     //Dedicated to Physics
     void FixedUpdate ()
     {
         controller.Move (horizontalInput * Time.fixedDeltaTime, /*crouch,*/ jump);
         jump = false;
     }
 }
 

gameobject.png shows how the Player is built. And platform.png shows how the Platforms underneath the Player's feet are built:

[1]: /storage/temp/127043-gameobject.png

[2]: /storage/temp/127044-platform.png

If you need the actual Unity Project to Resolve my Problem, you can Download it here.