Keeping a cat indoors is not cruel. It actually increases the cat's life expectancy.
He [an outdoor cat] has a significantly reduced life expectancy. Cats allowed outdoors have an average life expectancy of 5 years! (Indoor cats can live for 15-20 years) This is due to the higher risk of injury, death, illness, fights, etc.
Cats have been introduced as pets all over the world. They are not always fit to fight off predators and, as with most animals, have difficulty navigating with human life for example traffic.
In many places like Australia, cats devastate the native wild life. They're actually laws in most states preventing cats from being allowed to wonder outside at night. Some people resent cats so much they will actively attempt to trap and kill them. I've had this happen to my cats when I lived in the country.
Apart from injuries from accidents, there's less parasite issues, skin cancer from being exposed to the elements and from my experience, indoor cats tend to become more part of the household.
As mentioned, they do love to jump and need to scratch, so be prepared to make sacrifices to some furniture, even if you do provide scratching posts.
Also it takes time and patience to train them not to jump on kitchen benches and it's something that is hard to entirely break them from.
From a cat's perspective, jumping onto higher surfaces is natural. We have one cat that always jumps to the highest point of every room we have. We have baskets to store things and he likes to jump into these baskets. Or as mentioned you can buy custom cat homes that will incorporate perches and carpet.
In our case we live in an apartment and they can access the outdoors via our balconies. It's preferable they can see outside and it is better for them to be able to access the outdoors in some way, for sheer variety of experience. However it's not vital to their well being.
People often view cats as independent creatures, but they're not. They're completely dependent on their owners to care for them and to protect them. An indoor life is the best life a cat can have. If they are well fed, have their litter regularly changed and are given attention.
There's plenty of cat toys and you can use a simple string to play with cats and keep them entertained. Recently one of our cats has taken to our hair bands and will play with them. He stretches them and they then fling across the room, and he chases them. It's quite entertaining to watch.
As for aggression, the cat is less likely to be aggressive if kept indoors, as there's less threats to her territory inside.
Some interesting sources.
Your Cat—Indoors or Out - Mobile SPCA
Aggression in Cats - ASPCA