After a bit of experimentation this layout is one of the best I could figure out. I’m sure there could be some minor tweaks somewhere and if I figure out I will update it, but to me at this points it seems to be one of the best layouts possible.
The reason this layout will work so well is for a number of reasons.
Because you can’t fit an even amount of great towers around each side of your castle, you must place something else in the extra space. Here large towers are placed at the very end. Large towers are not nearly as strong as great towers and placing them at the end means your wall strength is not compromised in the middle where you need it at its strongest. They are placed at the end of the row of towers both on the inside row of towers and the outside row ensuring strength in the middle on both rows.
Guard houses have to be placed somewhere and most people from my experience will place them in groups on the 4 corners of their castle. To me this is a big mistake because usually only one row of towers protects them meaning it can be quite easy to gain access to them with a large force of catapults. Here players will for most of your guard houses have to destroy 2 rows of towers before they can get at your guard houses.
Ballista and Turrets
You should have your turrets heavily defended always and they should act as a serious line of defence for your keep if you walls are breached. Here they are placed in a rounded formation around the keep so if the enemy breaches your walls and gets near the keep they will have all 10 turrets firing on them which mow down troops like they are not there. The position of the turrets and ballista in this layout means no matter what side the enemy attacks you from they will have a minimum of 4 turrets and 4 ballista firing on them.
To even get near the gatehouse here the enemy troops need to walk a long path of killing pits which will probably kill most of them. Whatever remains will be killed by your turrets. Because the gatehouse is located at the rear of the keep your enemy will have to walk around the keep if they want to enter it. A force of pikemen or multiple captains in your keep will make it almost impossible to enter the keep from this side.
Moats and Walls
Moats are a much better choice between your towers than walls as if you towers are demolished by catapults your enemy still needs to dig up your moats buying you lots of time to kill them. Here the outside row of towers is surrounded by moats. The row of towers on the inside has walls between them. This is to stop catapults firing between your towers and hitting your guard houses behind them.
The key to any good castle design is to have a solid outside and a solid inside. Here both are true. The core of the castle is protected by a circle of great towers and also multiple layers of moats which provide a solid 2 layers of defence.
The two large towers on the inside row of towers can be replaced by smelters if needs be. Otherwise you can do without them.