For a first attempt, this is a very good effort, Bleda!
Playability: This map works perfectly well as a basic siege. Whilst it's nothing out of the ordinary, it is fine, works well and holds the interest.
Balance: A little weak here, as playing on the default normal setting when the game launches allows you to win pretty easily. There are a number of ways that you can siege this castle as an attacker. I noticed the back route cleverly disguised, but it seemed like pretty hard work to get there. So, I opted to break a few walls. The same result is there, a pretty clear route through to the keep. As a defender I thought the same, losing a couple of times before winning. I recommend this is played on hard setting fo those that prefer more of a challenge. Still, a good attempt.
Creativity: It's hard to give this a better mark, Bleda. It's not that what you have presented is bad, but there is no story to introduce you to the map and you have to imagine this is a straight forward siege or attck. Again, if this can be played both ways, say so. If it is specifically designed to be defended, or attacked, include this in a write-up, or on the downloads description page.
Map Design: The castle is fairly well constructed and is pleasing to the eye, if a little rough around the edges. The terrain looks okay, but could do with a little more modelling. Credit definitely given for the route through to the back of the castle though and there are some nice features. For a first attempt, it's damn good, and can only get better!
Story/instructions: Nothing really here i'm afraid. A good story is essential to gain the interest of a potential player, or even an introduction of some description. You don't have to make fictional worlds and mimic Tolkien or Scott-Rohan, but something to whet the appetite, as it were.
Still, despite the lack of story and the terrain which is a little on the bland side overall, it's a credible first attempt Bleda. You seem to have potential here and there are areas of the map that work well. It LOOKS like it's a working castle for a start. Keep at it, spend time exploring the editor tools and think of ways you can promote and interest others via a story or some kind of write-up. It all adds together in the end. Take heart from the fact that as a first map it is a very good effort and you can only improve from here, i'm sure! Well done!