Well, using the Basic D&D costs
100' of castle wall costs 5,000 gp
The gate and 2 towers around the entrance (Barbican) cost 40,000
Wooden Buildings cost 1,500 gp (2 story, 30x30)
Stone Buildings cost 3,000 gp (2 story, 30x30)
30 foot tall 30' round tower costs 30,000 gp
30 foot tall 20'...