Hey there, nice that you are trying out mapping. I think you got off to a very good start, looks very nice visually. Only thing I can think of in that regard is that you could try to do some more texture variation to break up the slabs of village texture. You already did it a little so it looks good enough, but a little more couldn't hurt. There is also the Ground Paint tool that is fun to play around with. Sometimes a hint of brown or grey here and there can give a map more of a natural or realistic look. Also the map is still very empty so you could place lots more scene props, but you were probably already planning that.
Designing a good siege map that is fun to play as well as balanced is very challenging. You really want different routes to the flag, but important enough choke points so that just camping the flag as defenders is not optimal. It is not entirely clear to me what routes there are available from the screenshot, but I see you have a bunch of backladders. For defenders this usually means that a group of people always have to camp the flag as it is hard to predict whether attackers will use this backroute, if they do they could easily do a ninja capture. For attackers these backladders are a very boring and long run from their spawn, during which they most likely won't have to fight anything. For defenders waiting at the flag is similarly boring. Perhaps you could try to integrate the secondary routes more with the actual castle so people might actually meet some defenders on the wall, instead of staying outside the wall until they are right at the flag area. I would advice you to remove the two furthest away backladders and perhaps work on another route within the castle.
Your map seems quite big. Keep in mind that defenders have a 30 second respawn timer and attackers a 5 second respawn timer, so you should be working off a running distance to the flag that is not too large. Of course the defenders spawn away from the flag usually as well and fighting and chokepoints completely change the calculation, but having the shortest route to the flag be around 40-50 seconds is usually a good idea. For starters you should probably put the attacker spawn points very close to the wall, to shave off like 5-10 s running already. One way to make longer attack routes work balance wise is by just placing the defender spawn points far away from the flag as well. Personally I really hate this though, if the flag is in danger and you spawn like a 30 second walk away, in my opinion that really sucks.
It is important that you keep in mind what you like in a siege map as an attacker and as a defender and that you keep in mind that the map needs to be balanced. Keep in mind that people find fighting generally more fun than running and waiting. If you keep all of that in mind you should turn out a good mapmaker.
Actually with the size of your map you could perhaps make it a conquest map. Fips said he would add 10 minute conquest maps. Conquest maps can feature multiple capturable flags, so you would be able to utilize the space you have much more than regular siege mode.
