private void PlaceTowerMarkers()
{
var placeableTiles = GameLevel.GetAllTilesOfType(LevelTileType.Placeable);
foreach (Vector2D tile in placeableTiles)
{
GameLevel.buildSpotMarkers.Add(new Billboard(new Vector3D(tile, 0.01f), Size.One,
ContentLoader.Load <Material>("TowerBuildingSpotMat"), BillboardMode.Ground));
}
}