map.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <link rel="stylesheet" href="css/map.css">

    <title>Document</title>
</head>
<body>
    
    <img id="home1" src="images/home1.png" onclick="checkLevel(1)">
    <img id="home2" src="images/home2.png" onclick="checkLevel(2)">
    <img id="home3" src="images/home3.png" onclick="checkLevel(3)">

    <div class="box bs1">start here</div>

    <img id="unlock1" src="images/1.png">
    <img id="unlock2" src="images/2.png">
    <img id="unlock3" src="images/3.png">
    
    <input type="button" value="back" class="bk" onclick="window.open('index.html','_self');">                   
    <script src="js/map.js"></script>
</body>
</html>