Edit in JSFiddle

var createMap = function() {
  /* global itowns,document,GuiTools*/
  const positionOnGlobe = {
    longitude: 2.3465,
    latitude: 48.88,
    altitude: 25000000
  };

  // iTowns namespace defined here
  const viewerDiv = document.getElementById('viewerDiv');
  const globeView = new itowns.GlobeViewExtended(viewerDiv, positionOnGlobe);

  globeView.listen(itowns.GlobeViewExtended.EVENTS.GLOBE_INITIALIZED, () => {

    var orthoLayer = new itowns.layer.GeoportalWMTS({
      layer: "ORTHOIMAGERY.ORTHOPHOTOS"
    });
    var planIGNLayer = new itowns.layer.GeoportalWMTS({
      layer: "GEOGRAPHICALGRIDSYSTEMS.MAPS",
      itownsParams: {
        opacity: 0.6
      }
    });
    var mntLayer = new itowns.layer.GeoportalElevation({
      layer: "ELEVATION.ELEVATIONGRIDCOVERAGE"
    });

    // adds 3 layers to the iTowns globe
    globeView.addLayer(orthoLayer);
    globeView.addLayer(planIGNLayer);
    globeView.addLayer(mntLayer);

    // adds a layerSwitcher control with specific configuration for ortho layer
    var layerSwitcher = new itowns.control.LayerSwitcher({
      layers: [{
        id: "ORTHOIMAGERY.ORTHOPHOTOS$GEOPORTAIL:OGC:WMTS",
        config: {
          title: "Orthos IGN",
          description: "Couche de Photos aériennes IGN",
          quicklookUrl: "lien/Vers/UnApercuRapide.png",
          legends: [{
            url: "lien/Vers/UneLegende.png"
          }],
          metadata: [{
            url: "lien/Vers/Une/MetaDonnee.xml"
          }]
        }
      }]
    });
    // adds the layerSwitcher control
    globeView.addWidget(layerSwitcher);
  });
}

Gp.Services.getConfig({
  apiKey: "ptlp78g6fvz9daz5mg2fztvr",
  timeOut: 20000,
  onSuccess: createMap
});

var infoDiv = document.getElementById("info");
infoDiv.innerHTML = "<p> Extension iTowns version " + Gp.itownsExtVersion + " (" + Gp.itownsExtDate + ")</p>";
<body>
  <div id="viewerDiv">
    <button id="tiltButton">
        jsFiddle bug - click on this text to activate tilt (ctrl + left click)
        <br>
        Do not integrate this button to your own non-jsFiddle pages
    </button>

  </div>
  <div id="info"></div>
</body>
html {
  height: 100%;
}

body {
  margin: 0;
  overflow: hidden;
  height: 100%;
}

#viewerDiv {
  margin: auto auto;
  width: 100%;
  height: 400px;
  padding: 0;
}

#tiltButton {
  background-color: white;
  position: absolute;
  top: 20px;
  left: 50px;
}

#info {
  padding: 5px;
  width: 100%;
  height: 20px;
  font-family: "monospace";
  font-size: 10px;
}