Checked the map finally. To answer your question regarding balance:
I examined the battle in two fronts. I did a small analysis of the things overall.
First, the spawns:
The castle from the point of view of the attackers. The routes they will follow generally. (you might want to test that spawnpoint, I just spawned there as an attacker. Maybe it should be entry point no 33, not 32.)
Now from the point of view of the defenders. Red lines are infantry, green circles are archery spots and blue lines are cavalry.
The scenario and analysis:
The attackers will simply rush to the both of the gatehouses and will open the gate. Since their spawns are very close to the walls and much more concentrated, the chances for them are they might break through the weak resistance of the defenders. If this happens, they will open both of the gates. There is a chance that this map can finish it's first round very quickly. But defenders have very good reinforcement paths to the gatehouses, so the gates might not stay open for long enough. If the defenders prevail, the chances of the attackers won't be much. But there are side/backladders for that, and the defender's good reinforcement paths also has a natural weakness. Once they are in gatehouse spots, returning back is no easy feat; which is also a con for the attackers as well. The battle will be in two fronts at minimum. One in the gatehouses, one in the other places. The gatehouse front will affect the other fronts. Yet, there won't be a way of force transfer to the other fronts. İronic, simple and pure brilliant.
That is how it seems when we only look at the gatehouse front. However things might not work as intended, as that tight underway has it's own problems without the gates and even without the water. I once played a fire&sword siege map including a tight passageway like this (with no gates and water), it was no easy feat to break through that passage; considering the enemy was welcoming us with a shield&pike wall. In this map's passageway the water also slows you down to be easy prey to enemy missile, and gets you drowned obviously. The defenders can stay in water, and step back for a while to breath up. Which means they can meet the attackers in a position where attackers can't breathe and stop them there simply by holding a simple shieldwall. As long as they hold their ground for 5-6 seconds, the water will do the job. Not to mention the attackers will have to go full force to hope to break through that underway, which means they might not maintain to keep the gatehouses open. As 10-20 defenders can stop a force of 40-50 attackers there in the current conditions, the defenders can spare more men to the gatehouses. Moreover, the defenders can still win with ease if they don't care about the gatehouse and camp the underway.
There is one more factor to take into consideration. Even if the attackers break through the defence of the underway, they will still have to fight to get to the flag a bit and they need to hold it afterwards to get down the flag hopefully. All in all they will have a hard time, even after they are close to the victory. I think this is a bit unbalanced.
My suggestion would be, doing some rastical changes to the underway passage. Maybe add a couple of objects to cover a part of the watery areas, so the half of the attackers can go while they can still breathe and others simply passing under the water to increase the concentration of the force. The length of it also might be shortened. Last but not least, some objects to give the attackers some cover from ranged might be useful too; though I don't think that might work as intended. The decision is up to you of course, these are the only ways that I can think of. Just not add another-easier way to get into the castle, as all of the attackers would have use them and make the underway (the epic feature of this map) will be pointless.
Here are a couple of screenies. I especially liked how two gatehouses connect to each other. And the broken bridge is a complete trap
All ladders/gatehouses are working fine by the way. There is only a problem with backdoor. It is impossible to jump that much of a distance. I'd suggest making it much much more easier, as attackers will suffer in this map already