Leonardo Da Vinci in in's Creed the Brotherhood!!?

I'm stuck. I just met with Leonardo and I got the double blade from him (that's all I had money for). Now I can't go burn down the three remaining Borgia towers because those areas "aren't available yet" and the ony explanation mark I need to get to is in one of those areas and I have no idea what to do. I'm in sequence 4 memory 8 I guess but I'm definitely stuck. Please help!