// Move the object around with the arrow keys but confine it
    // to a given radius around a center point.
    var centerPt: Vector3;
    var radius: float;
    
    
    function Update() {
        // Get the new position for the object.
        var movement = new Vector3(Input.GetAxis("Horizontal"), 0, Input.GetAxis("Vertical"));
        var newPos = transform.position + movement;
        
        // Calculate the distance of the new position from the center point. Keep the direction
        // the same but clamp the length to the specified radius.
        var offset = newPos - centerPt;
        transform.position = centerPt + Vector3.ClampMagnitude(offset, radius);
    }

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    public Vector3 centerPt;
    public float radius;
    void Update() {
        Vector3 movement = new Vector3(Input.GetAxis("Horizontal"), 0, Input.GetAxis("Vertical"));
        Vector3 newPos = transform.position + movement;
        Vector3 offset = newPos - centerPt;
        transform.position = centerPt + Vector3.ClampMagnitude(offset, radius);
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    public centerPt as Vector3

    public radius as single

    def Update():
        movement as Vector3 = Vector3(Input.GetAxis('Horizontal'), 0, Input.GetAxis('Vertical'))
        newPos as Vector3 = (transform.position + movement)
        offset as Vector3 = (newPos - centerPt)
        transform.position = (centerPt + Vector3.ClampMagnitude(offset, radius))