For the introduction introduced the play, bring up the meaning of betrayal and mention that Julius was betrayed. But don't explain it all there, you can go more into detail about that stuff in the following paragraphs. For the concluding paragraph you can briefly summarize what you had written, but don't bring up any new information at all.
The green girdle represents his survival and failure. "A man may hide his misdeed, but never erase it." He says it's a code of honor
