Project Setup & the Player Controller

A platformer starts with the one thing every level depends on: a player that can move and jump reliably. In 2D Unity, that means Rigidbody2D and a ground check.

using UnityEngine;

public class PlayerController : MonoBehaviour
{
    public float moveSpeed = 6f;
    public float jumpForce = 12f;
    public Transform groundCheck;
    public LayerMask groundLayer;

    Rigidbody2D rb;
    bool isGrounded;

    void Awake()
    {
        rb = GetComponent<Rigidbody2D>();
    }

    void Update()
    {
        isGrounded = Physics2D.OverlapCircle(groundCheck.position, 0.2f, groundLayer);

        float move = Input.GetAxisRaw("Horizontal");
        rb.linearVelocity = new Vector2(move * moveSpeed, rb.linearVelocity.y);

        if (isGrounded && Input.GetButtonDown("Jump"))
        {
            rb.linearVelocity = new Vector2(rb.linearVelocity.x, jumpForce);
        }
    }
}

The pieces

• Rigidbody2D (not the 3D Rigidbody) handles gravity and movement for 2D games, set its Body Type to Dynamic in the Inspector.
• Physics2D.OverlapCircle(point, radius, layerMask) checks whether any collider on groundLayer overlaps a small circle, a cheap, reliable way to ask "is the player standing on something?" without relying on fragile collision-event timing.
• groundCheck is an empty child GameObject positioned at the player's feet, purely there to give the overlap circle a position to check from.
• Input.GetAxisRaw (unlike GetAxis) returns exactly -1, 0, or 1 with no smoothing, snappier and more predictable for platformer movement.
• Only jump when isGrounded is true, otherwise the player could jump endlessly in mid-air.