"Stronghold 3 Gold" doesn't support "ladderman" and "siege tower' in Map Editor.
Without ladderman and siege tower, I think that creating "castle defense map" is meaningless.
Anyway... instead of castle defense, I created mountains to enjoy an ambush.
Enemy Troops are 350 Men at Arms and 50 Swordsmen.
They will pass through a narrow path between the mountains.
When Enemy Troops nearly reach the end of the narrow path, "Human Player Archers" will appear suddenly.
The archers will use fire arrows and you can use "Rolling Logs" whenever you want to use.
I didn't put "win or lose conditions".
Just enjoy the fire arrows and rolling logs are killing enemy troops.
And move units to the top of the mountains.
You can edit my map in Map Editor as you like.
You can upload the map freely that is based on my map and you edited.
In Scenario Editor, I used "Invasion" Action because I had no other choices.
Archers that are placed in line looks better, so I placed AI archers in line.
Human Player Archers don't wait and attack enemy immediately,
so they have to be AI archers before an ambush starts.
I tried to use "SET_AI_PLAYER_ALLIED_TO" Action but it ocurred an error and I couldn't use that.
If "SET_AI_PLAYER_ALLIED_TO" Action worked, I could change AI archers to Human Player Archers.