Premise liability

[Nan Jredy]

Backside of my rental property is uneven and slopped with a retaining wall that is 4 feet high. I do not have anything to prevent the tenant from straying into the back of the house. How can I prevent someone from entering that area other than building a fence and locking it? I am trying to prevent personal injury issues. Should I issue a written warning/notice to the tenant to not to enter the back of the house? The tenant has not business to be at the back of the house, but I want to make sure I am doing the right thing.