What does your lease say about late fees?
If you want to push things to the point that you're in court, you'll be free to argue to the court, to the extent that the facts would support the argument, that the late fees aren't described in the lease and thus cannot be asserted (they are a matter of contract), that the landlord's past acceptance of rent without your ever having paid late fees and thus the provision should be deemed waived, or that the late fees are excessive. We're not in a position to assess those defenses, or any others, as we don't know the relevant facts.

