In general, it's allowed except where it's specifically prohibited. Examples of prohibited defenses include:
- harming contact inside the other robots' frame perimeter
- Preventing opposing robots from crossing (or presumed crossing) the defenses into/out of your courtyard
- In the secret passages, the owner of the passage has right of way.
- in your own courtyard during the last 20 seconds
This may not be complete; see the game manual for details.