It is really hard to tell anything based on a handful of photos but I'll do what I can.
First, I'd echo what Gumpy said, the courtyard is just too open. However I think it can be simply solved by placing various carts and barrels and whatnot. So once you are done with detailing it may be a bit better.
Second, think very long and hard about map flow. If you can't get a group of people to beta test this then run through it yourself and try to imagine everything and anything that might happen. Most importantly you need to analyze flow to the gatehouse, and flow to the flag. Ideally the gatehouse should be harder to attack than the flag, but once you get the gate open the flag should be significantly easier to attack than with the gate down. Hailes is a good example of gates used correctly. Mahdaar (though I think it is actually a wonderful map) is an example of gates used poorly. In Hailes the gate is very hard to take, but when taken it really opens up the castle for attackers. However it doesn't guarentee a win either, a hard fought D can get the gate closed.Mahdaar, on the other hand, is stupid easy to take and doesn't honestly buy the attackers much anyways, since they need to climb stairs to get up to the flag.
Next thing is doors. Having a sally door is a good idea, but you want to make it a certain way. Turin is an example of a sally door done right. Rudkhan is an example of a sally door (and almost everything else) done wrong. On Turin the door is out of the way, and provides easy access to the flag if broken. However, it is VERY easy to defend with one or two archers. Always bracket breakable doors with two sides of fire. Also make sure it is not in front of the attackers at spawn. Do not make it like Rudkhan and have the sally door be the primary point of assault. It will way over-power horses, and basically boil the map down to a single avenue of attack.
Finally, the siege engine. The siege engine is what is what should set the pace of the siege itself. You have a pre-siege engine part of combat and then two or three post-siege engine parts of combat. During the pre-siege engine combat you mostly have some skirmishing happening on the ladders while attackers probe for weaknesses, and you also have a **** ton of archery going on to try to slow down the advance of the siege engine. Once the siege engine is at the wall you have combat running through pushes where the attackers send a large wave forward and try to establish themselves wherever that siege engine leads (such as the gatehouse on Mahdaar, and the siege room on Turin) and then try to push outwards into the castle, or else get pushed back out of the castle.
That's just some general advice. Here is some specific advice from what I can see.
Move the siege tower closer to the spawns, if possible. A siege tower should be the primary mode of attack and so should basically be right in front of the attackers. It will also concentrate a bit more of the attack near the gatehouse. Remember, people don't play this game for the fantastic movement system, they play it to fight. The more fighting, and the more intense the fighting, the better. Of course, I just realized that I'm assuming the spawns for attackers were across the bridge, but that doesn't actually seem logical to me. So ignore this if the spawns are close to the siege tower already.
Be careful with defender spawns. That top of the keep looks cool and all, but don't put defender spawns up there. That is way too far away, and will be frustrating for defenders.
And generally, be careful of size. Size isn't really a balance issue so much as a fun issue. People talk all the time about how an oversized castle is hard for attackers to siege, but that is entirely up to the layout of the map. A big map can be just as hard to defend as some other big map is to attack. The real problem with size is simply that it scales combat back. You spend more time running than you do fighting. The problem also isn't always with the physical size of the map, but rather the overall compactness of the map. Turin is a decently sized map, but it is very compact overall, with lots of fighting being focused into small rooms and corridors. That is why Turin is such an intense and fun map, the combat is very, very focused.
I suggest moving some of those back ladders closer to the main front of combat. You don't want 1 or 2 people fighting at each ladder or entry point, that is going to get stale. You want big 6 to 7 groups fighting eachother at each entry point. Again, think about Turin. Imagine if one of the two tower ladders on Turin was moved all the way around to the back wall. The combat would get spread out all over the place, with no real defined front. Rudkhan castle suffers heavily from this, until the door is broken. You have people running up the siege ramp, climbing the siege tower, and climbing two very widely space ladders. The combat is strung out across the whole wall. That would be alright if there are 200+ people playing, but not if you only have 50 - 60 people playing. (of course I'm making an assumption again. I'm assuming you are designing this for the GK siege server since that's where you always seem to play)
I hope this helps a bit. BTW, this is Isord from GK. Hopefully we'll get to battle on the map soon. It looks like it has potential, and no matter what anyone says now or how much work you put into it now, it'll need to be play tested before anything can really be said about it.