The Romans didn't generally wall their cities until the 3rd century, and even colony towns were often unfortified. Boadicea destroyed undefended towns and mostly slaughtered defenseless civilians. When it came time to overwhelm a much smaller Roman force, pllllt, she was gone.
I really enjoyed reading Caesar's Commentaries on the Gallic Wars years ago, particularly the last book in which Vercingetorix was surrounded with a good sized force on a hilltop fort; Caesar built pallisades all the way around it, and knowing that all of Gaul was on its way to relieve Vercingetorix, built another set of pallisades on the outside, a sort of doughnut. That guy sure had a set of stones. ;')
Well, she certainly didn't do any worse against the Roman Legions than any number of other indigenous Euros. I guess the Germanic tribes had the best track record against the Romans, but I am not very well read on this subject.