Can I sue my nasty ex-landlady?

You could sue her, but before doing that I would go directly to her first and demand the money. She did, after all, say she would give it back to you. So try to get it from her first. I say this because when you take someone to court, no matter how badly you've been screwed over by this person, there are no guarantees that you'll win. So I think you'd have a better shot of going straight to the source. Annoy and harass her if you need to - it is your money, and she did say she would give it back to you. Maybe even threaten her-tell her if she doesn't give you your money back, you will sue her for double the amount (that is customary with unreturned deposits) as well as the amount she cost you when she illegally entered your apartment. At this point, you haven't really done all you could to get the money back from her directly - just be more persistent, call her every day, even go to her tenants and tell them this story if you have to so they'll know what kind of person their landlady is. Try all that before thinking about getting the courts involved.