Hey, nothing is too ambitious! Don't talk yourself down! The fact that you're even trying puts you about 90% of other artists.

I'd like to speak a little bit in hopes that you'd gain some value from it. You asked up above, "without high fantasy elements like dragons and magic, why write this story as opposed to any other in that same world?" I think that a possible answer to that rolls together with your statement about the crusader, that more extreme version of yourself. So you ask, "why does one siege matter from another, if they're all just low-tech slugfests?" It matters because of what the characters see and do, of course. Is there betrayal? Heroism? A glimpse of something suspicious between moments of combat?

They say, "Is this the most exciting time of your character's life? And if it isn't, why are we following them?" And you talk about how you're intrigued by politics and intrigue, as well as the disillusionment of the crusader. Since we're all aware that the crusades were really quite a disappointing and dishonorable time, I think you're coming onto a great premise for this story.

A crusader who has traveled an extreme distance from his home is now struggling with his faith, the foundation of himself, in a foreign land. He looks for meaning in his commanding officers-- a pious leader who guides them, a paragon of morality. But in the crusader's time between battles and skirmishes, he slowly comes upon the tapestry of sin that has been woven by this commanding officer. The strain of our crusader's crumbling identity leads him to make decisions he once thought impossible, and the dynamic of himself is permanently altered as he is enveloped in the dark intrigue of the things he discovered, so far from home.

That's a pretty basic pitch, there's a lot of room to tweak it and change it around. But you can see that your idea here really isn't far from being something very good; in fact, it's quite close to being something excellent! You can definitely do some good stuff with this. Toy around with disconnected scenes, just trying out characters and locations and ideas. Write the final battle first, kill the main character in non-canon passage. Do whatever. Invest yourself in his exploits and his discoveries. I think that a lot of the confidence in creating a story comes from the place where you identify with the people you're writing about.

Anyway, I'll stop myself from rambling. But I hope you gained something from what I had to say. Don't doubt yourself! Try it out, experiment! You're already an artist in your own right, no need to feel insecure just because it's a new medium.