Xslt 1 convert non-unique elements to attributes for grid

I have an XML file with this structure

<levels>
  <level id="0" qd="NE">
    <gate>99</gate>
    <zone>2</zone>
    <laydown>4</laydown>
  </level>
  <level id="0" qd="SE">
    <gate>1</gate>
    <zone>6</zone>
    <laydown>1</laydown>
    <laydown>2</laydown>
    <zone>5</zone>
    <laydown>3</laydown>
  </level>
</levels>

And I need to convert it to something like this to display in a grid

<level id="0" qd="NE" gate="99" zone="2"  laydown="4">
<level id="0" qd="SE" gate="1" zone="6,5" laydown="1,2,3">

The order of the comma separated list is not important for me. I am an xsl/xslt newbie so any annotation will be helpful. I work with VS2013 which I gather only supports XSLT 1. Previously asked in the title for XSLT 2 so thought it best for this to be a separate question.