If you have Saturn near the top of your chart, visible to the world, then there is a good chance that you will be ambitious, in the sense of there being some sort of mark you will need to make on the world, that helps define who you are. We are here to incarnate, to bring spirit into matter, and Saturn is the planet that forces us, sometimes painfully, to do just that. So it is not just an ego thing: by putting your mark on the world, you are giving birth to yourself. As I wrote the above, someone with Saturn in the 10th House asked me for a reading. So I'm probably onto something :)
But Saturn can also be limited. He can have his ears closed to the outer planets, indeed he may try to boss them around, and that is always messy. Saturn is also the Protestant Work Ethic, that tells us to define our worth by how much we work and how much we have visibly achieved. So if you have a prominent Saturn, and you have a drive to be self-aware, then I think there is often a process of disentangling to be done after you have made your initial mark. You may have a painful period where the old is no longer working so well, so how do you define yourself now?
One way of looking at this is that our true nature is formless, while anything we put out into the world by definition has form, has Saturn, so it cannot fully express who we are. And who we are is, hopefully, going through a deepening process all the time. This is why poets pay such attention to the words they use. Giving exact expression to the inner world is difficult, and always an approximation.
And then there is the good old ego. It comes out one way or the other, whatever our chart. In the case of a prominent Saturn, you can ask yourself how you feel in those interim periods where you have no projects, and the world is not noticing you? How brittle are you if someone critiques whatever it is you use to define yourself? How threatened are you by your rivals? We can learn a lot about ourselves from these places, precisely because they are uncomfortable.
Then there is Saturn in the bottom half of the chart. I am being schematic here, because it is never this simple. But simplicity can be illustrative. I have Saturn hidden away in the 4th. I'd say that it is not in my nature to keep myself very busy, to do lots as a way of defining myself. I do less, but what I do comes after a lot of cooking and tends to work well, and there is not too much of 'me' in the way of it. Saturn has, to a large extent, become the guardian of my inner life instead of, as he started out, the denier of its value. This has been the big Saturn battle for me.
It is a difficult and elusive placement, because Saturn is naturally much happier with outer, visible things that can be measured. How do you measure inner change? Well, you don't. Only you know it, though you may also get good feedback from others sometimes about who you are, rather than about the great things you have done. When I was younger, I was always trying to prove something about myself through what I did, and it rarely worked very well. In fact, it often hardly worked at all. This is because my Saturn does not belong primarily out in the world.
The challenge with the extraverted Saturn is not to let your sense of who you are get too tied up with what you do. It will limit your awareness of your own fluid depths, and keep others at a distance. With the introverted Saturn, I think the challenge can be to learn to value yourself simply for who you are, and not indulge a feeling of worthlessness on the world's terms. This Saturn, once he is on side, can give you self-mastery, gravitas, depth.
Mythologically, Saturn devoured his children at birth, jealous that they would overthrow him. In the same way, Saturn can lead us to doubt whatever we have genuinely achieved, telling us it is not good enough. He is very strong in this form in the culture at large, because that is how large societies tend to control themselves. Original Sin was in this respect a great wheeze for controlling people. Saturn goes both ways: he helps us achieve, helps us incarnate. But then denies the value of that.
