What should I say other than "Il est un"?

I'm doing a french project on The Count of Monte Cristo and my sentence says "Dantes est arrete parce qu'il est un traitre." But my french teacher says I can't say "il est un" so what should I use instead? Also, if I were saying, La prison ou Edmond est enfermer (the prison where Edmond is locked up) could I use enfermert? instead of est enfermer?