Most frequent cause this happens is because you did not have enough gold in their city. This can happen for a number of reasons:
- You forgot to put gold there at all, and knight unassigned.
- You had gold, set marshal, but was attacked before opening. The attacker stole the gold. You put more gold there, but did not realize the knight had unassigned in the meantime. When they open, the knight is not on duty.
- You forgot to put marshal on.
- You put marshal on, but the system did not register it fully. (This happens with putting gear on a knight or embedding jewels in gear as well). If you refresh before opening, you will note that the knight is not actually assigned (or gear or jewels are not on). If you don’t refresh but just open, you will have no marshal since it did not register with the system even though it showed the knight assigned in the knight’s hall.
- You were using a reinforced knight as marshal instead of assigning their own knight.
The reinforced knight will only stay as long as some of the troops sent with the knight are still alive and defending. For this reason, it is best to send a reinforced knight or heroes with 1 wagon, in addition to other troops if desired. Wagons are the last troops to be killed, only after all the fighting troops are dead. So this will keep the reinforced marshal on duty.
To keep in mind:
Sometimes players send reinforcements with a T6 or T5 or other high tier troop instead of a wagon, though. If I see a single T6 in someone’s city, I’ll send some arcane archers along on an attack to kill the T6. Often some of the reinforced heroes are gone on my next attack as a result. If there was a reinforced knight too, then they will be gone as well and the city will in effect be unmarshalled.
The safest way to open with your own knight on marshal is to make sure you have gold, assign your knight, and refresh to confirm it is there before opening. If someone attacks you and steals your gold before you open, add gold, unassign knight and reassign knight, then refresh again. If the knight is still there, open.
The safest way to open with someone else’s reinforced knight acting as marshal is for them to send the knight with at least 1 supply wagon/convoy/caravan. It's also not a bad idea to assign a lower marshal of your own. The stronger knight should take over, but if it falls off duty for some reason, you at least have some protection.
It is also a good idea to have someone you trust send a test hit of 1 troop before you announce that you are open. That way, if there is a problem with your marshal, you will know about it and be able to fix it before you are zeroed.