If you are not restricted to C# < 3.5. I would use
LINQ to XML[
^] to that type of query. Very easy to use.
XElement xml = XElement.Load(@"c:\xml\data.xml");
var result = from units in xml.Element("ARI").Elements("Unit")
from city in units.Elements("city")
where city.Attribute("ID").Value == "1"
select city;