So, the game is named castles, but at first I was a little disappointed actual castles don't give the advantage they supposedly should. In fact for a time I even preferred open field battles - as there I quickly learned how to beat forces outnumbering me over triple times, while castle defense was nowhere that successful more often than not.

But, my designs of castles keep evolving, and after several paradigm shifts I come to understanding how to game the game, and developed this castle - as shown in attached picture.
Finally I'm able to reliably rely on the castle as solid defensive structure. I found it interesting and thought, can as well share :)

The interesting bit is, it is actually totally unreachable by AI on its own - if you don't want to fight just keep anyone in the inner castle and the battle will time-out in "deadlock" long before even knights will reach the walls.
If you do want to kill the attackers however, you can do that too, either place your own knights out of the castle and pick on they archers and siege engines from behind while their knights run in to your archer fire, or, if you do not have knights, extend one archer somewhere in the outer field of towers so it can make contact early enough (that poor fella will likely die though, small square towers seem to not give much protection against ground level archer fire, but at least according to manual those are (slightly?) faster to build so I used them for the territory claim field).

One thing I miss in it is connector walls to extended, now free standing, tall round towers, ability to be easily mobile to and from those over top of the wall would be great, but, it would need another 16 peaces... and this is already maxed out as you can easily guess.

But anyway, as of now no infantry yet had to successfully climb even my outer wall, and their knights seem unable to map patch trough the 5 gates, they run into wall an stuck there...
...yes, this castle is feeling a bit cheat-y even ;)

I'm sure other players have found some interesting castle designs as well, could be fun to see.

Nice (and yes, a bit cheat-y) :) Thanks for sharing.

My own castles tend to be much like your inner design, without all the fancy moatwork. They don't work well, of course! For a little more advantage I add outer, low walls to slow enemy infantry down, just in range of my archers in the towers. It's "fine", but as you say, the defensive advantage of castles is surprisingly low.

This is absolutely brilliant!

I've been playing this game since the mid 90s. I have always just used the standard design castles because I didn't see the point.

Ill be making one of these super structures next time I play :D