XML/XSL Multi Level Grouping examples wanted

Hello Experts;

Trying to learn XML and working on creating a multi-level grouping.  I have read all I can find on the Muenchian Method and think I understand a single level grouping.  However I can't seem to grasp the multi-level grouping.

Does anyone know of examples available for viewing that would perhaps explain this.   I have read the Jenni Tennison articles but it's just not clicking.

Here is what I am trying to do

CATEGORY
     OWNER
          PROJECT
          PROJECT
     OWNER
          PROJECT

CATEGORY
      OWNER
          PROJECT
      OWNER
          PROJECT


Here is my XML:

<?xml version="1.0" encoding="utf-8"?>
<?xml-stylesheet type="text/xsl" href="projects_2.xslt"?>
<projects>
  <project>
    <category>healthcare</category>
    <name>Medical Building Expansion</name>
    <owner>Christiana Care Health System </owner>
    <descp></descp>
    <iscomplete></iscomplete>
    <completiondate></completiondate>
  </project>
  <project>
    <category>healthcare</category>
    <name>Cancer Care Center</name>
    <owner>Christiana Care Health System </owner>
    <descp></descp>
    <iscomplete></iscomplete>
    <completiondate></completiondate>
  </project>
  <project>
    <category>healthcare</category>
    <name>Emergency Room Expansion and Renovation</name>
    <owner>Atlantic General Hospital</owner>
    <descp></descp>
    <iscomplete></iscomplete>
    <completiondate></completiondate>
  </project>
  <project>
    <category>senior care</category>
    <name>Methodist Country House Hall One Conversion</name>
    <owner>Peninsula United Methodist Homes, inc.</owner>
    <descp></descp>
    <iscomplete></iscomplete>
    <completiondate></completiondate>
  </project>
  <project>
    <category>senior care</category>
    <name>Cokesbury Village Christiana Court East</name>
    <owner>Peninsula United Methodist Homes, inc.</owner>
    <descp></descp>
    <iscomplete></iscomplete>
    <completiondate></completiondate>
  </project>


Here is my current XSL which gives me a single level sort:

<?xml version="1.0" encoding="utf-8"?>

<xsl:stylesheet version="1.0"
    xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:key name="categories" match="project" use="category" />
<xsl:template match="/">
  <xsl:apply-templates />
</xsl:template>
  <xsl:template match="projects">
    <xsl:for-each select="//project[generate-id(.)=generate-id(key('categories',category))]">
      <xsl:sort select="name" order="ascending" />
      <h3>
        <xsl:value-of select="category"/>
      </h3>
      <table border="1">
        <tr>
          <th>Owner</th>
          <th>Project</th>
        </tr>
        <xsl:for-each select="key('categories',category)">
          <xsl:sort select="owner" />
          <tr>
            <td>
              <xsl:value-of select="owner" />
            </td>
            <td>
              <xsl:value-of select="name" />
            </td>
            <td>
            </td>
          </tr>
        </xsl:for-each>
      </table>
    </xsl:for-each>
  </xsl:template>
</xsl:stylesheet>


Thanks for any advice or examples!