The area where I live is extremely lethal for a domestic cat. There are scorpions, tarantulas, poisonous snakes, dogs, foxes, eagles, vultures, larger wild cat species, and even sometimes leopards. My cat (Bhutu) only has experience being outside back in the city where the streets only had weak and unfed stray dogs as the only danger to cats (which still was a significant danger).
Our house is a huge two storey house in which we only live in a part of first floor. Very recently, Bhutu managed to get onto a sloping, slippery metal roof. Had she fallen from this height, it could have been fatal. We always keep all windows closed and most of them are the olden days wooden windows with no glass. So the light always has to be on, resulting in an increased electric bill.
Moreover, there is no door at the front entrance of our part on first floor. Instead there is a huge collapsible gate through which a cat can easily pass.
In the past we've always kept her on a leash to restrict her free movement but the cat doesn't enjoy this. We have also tried other methods like putting a large object that the cat is fearful of (like teddy bear) next to the entrance but she always has the tendency to venture outside.
How can I ensure that my cat will stay inside without the requirement for a leash?