borrows: (for anyone who doesn't know what they are and their purpose) they are small tunnels made by soldiers to pass under the castles walls or enemy front line.
for haven and hearth better to make it a super expensive tech that also needs a special object (dk maybe smthing like a drill) to make one.
defense: all borrows are connected {q. what do you mean?!?!? a. if u make a borrow under your wall and reinforce the walls with metal then no one can pass and u can see if smne is approaching (like u can see mines at minimap)