("castle_start_siege",
[
(this_or_next|party_slot_eq, "$g_encountered_party", slot_center_is_besieged_by, -1),
( party_slot_eq, "$g_encountered_party", slot_center_is_besieged_by, "p_main_party"),
(store_relation, ":reln", "$g_encountered_party_faction", "fac_player_supporters_faction"),
(lt, ":reln", 0),
(lt, "$g_encountered_party_2", 1),
(call_script, "script_party_count_fit_for_battle","p_main_party"),
(gt, reg(0), 5),
(try_begin),
(party_slot_eq, "$g_encountered_party", slot_party_type, spt_town),
(assign, reg6, 1),
(else_try),
(assign, reg6, 0),
(try_end),
],
"Besiege the {reg6?town:castle}.",
[
(assign,"$g_player_besiege_town","$g_encountered_party"),
(store_relation, ":relation", "fac_player_supporters_faction", "$g_encountered_party_faction"),
(val_min, ":relation", -40),
(call_script, "script_set_player_relation_with_faction", "$g_encountered_party_faction", ":relation"),
(call_script, "script_update_all_notes"),
(jump_to_menu, "mnu_castle_besiege"),
]),
The red part checks your relation. Comment that out.
Edit: in module_game_menus