Viewing a geojson as 3D in cesium

You need to extrude polygons:

entity.polygon.extrudedHeight = entity.properties.your_height_field

You may need to loop over records, for example:

    var p = dataSource.entities.values;   
    for (var i = 0; i < p.length; i++) {
         p.polygon.extrudedHeight = entity.properties.your_height_field

The accepted answer is wrong. Here is the correct example:

<div id="cesiumContainer"></div>
<script>
    var viewer = new Cesium.Viewer('cesiumContainer');
    var dataSource = Cesium.GeoJsonDataSource.load('buildings.geojson').then(
        function(dataSource) {
            var p = dataSource.entities.values;
            for (var i = 0; i < p.length; i++) {
                p[i].polygon.extrudedHeight = 15; // or height property
            }
            viewer.dataSources.add(dataSource);
            viewer.zoomTo(dataSource);
        }
    );
</script>