The Secret passage card may be used by other players on some circumstances:
Rulebook page 14 wrote:
: A Secret Passage card can be played by any Player to help the Uprising cancel the Attack Penalty [of fortifications].
While for other effects the Rulebook seems to imply that you can play it on your lords only, italic mine:
Rulebook pharagraph 10.2.1 wrote:
When played during movement, the Player can utilize one of
the following effects for his own Lords and Troops:
- Escape from a Besieged Stronghold or Fortified City
- Move in Heavy Rain
- Avoid a Battle
Then for this circumstance the rulebook says you can play it on "an attacker". It does not specify that it must be your Lord:
Rulebook pharagraph 10.2.2 wrote:
Played during a Battle
When a Secret Passage card is played during a Battle, the Player can:
• Attack from the Tunnels
Allows an attacker to attack a Stronghold or Fortified City
from the tunnels, cancelling all Attacker Penalties.
While for Assassinations the player needs to play on his own Lords.
Rulebook pharagraph 10.2.3 wrote:
During any phase
• Avoid Assassination or Ambush - A Lord can ignore an
Assassination or Ambush attempt if he plays a Secret Passage
card after an Assassination (10.3) or Ambush (10.5) card has
been played. Both the Secret Passage and the Assassination or
Ambush cards are then discarded.
To simplify this, and to allow more negotiations, we houserule that you can play Secret passage on another player's lord if you are on the same village or during any battle and uprising to ignore the defenses (since the rulebook allows it).