This is the menu choice:
("recruit_volunteers",[(call_script, "script_cf_village_recruit_volunteers_cond"),]
,"Recruit Volunteers.",
[
(try_begin),
(call_script, "script_cf_enter_center_location_bandit_check"),
(else_try),
(jump_to_menu, "mnu_recruit_volunteers"),
(try_end),
]),
Now open module_game_menus.py and find: trade_with_merchants
Above it add the above code.
This should make it work for both castles and towns since there is nothing checking whether you are at a castle or a town and both castle and town menu's are in the menu for towns.