It's not bad at all for such an early build and serves its purpose reasonably well. One improvement I'd recommend when you have the materials . . . make a safely fence around either the entire 'keep' or at least the door area out to a distance sufficient that creepers won't automatically explode should you find one waiting against the fence when you exit the structure. A great many of my survival structures feature such a fence protecting their major entrances.
Well, this is a start... Also, that's just a tower / turret
Anyways, I decided to build this in Creative on my practice world and see what I could do to make it look better. I came up with this (spoiler'd for length):
New front of the tower.
With the arrow slits
Interior. You can see where the floor is supposed to go. I didn't do any furnishing except on the first floor for the sake of not having to take ore pics / spamming this thread for my unasked for tweaks.
Texture pack is Conquest with Optifine + 3d models (basic pack)