Unrully neighbor

My neighbor moved in 6 months ago. He cut down, and poisoned the ivy. Which was about 4 feet thick and on both parties property. He then took down the fence and put up a new one exactly the same. Now he wants me to pay half of it. The ivy was also serving as a retaining wall. Now the wall is eroding and he says I have to build a retaining wall.
My questions are 1. Is ivy illegal to grow on your private property?
2. Who is responsable to build the retaining wall? The property owner that is 5 feet higher, or the property owner that is lower by 5 feet. (The cliff is eroding into a hill.)
3. Am I responsable to pay for half of it, or not?
4. Can i get my neighbor to replace the landscape he killed and poisoned that was on both properties? (He went 3 feet onto my property to do so.)

He cannot cut or destroy anything on your side of the property line without your permission. If the fence sits on the middle of the line, both must agree. So keep your fence on your side and if need be sue in small claims for any costs or damages he may have caused you.