If they were going to sneak in, they would need to decide on a route. For that, Koutarou's knowledge from the past came in handy. Old castles had parts that required them to be connected to the outside. The most convenient of which was the aqueduct.
Castle from that age were required to function as fortresses. Therefore, they had to be able to get water from the outside or they wouldn't be able to last in a siege. In the case of the castle of Sariachal, that came in the form of an underground aqueduct.
“How does it look, Clan?”
Rokujouma No Shinryakusha V19 p153.jpg
“We're in luck! The facilities from 2,000 years ago are still here!”
”Alright, luck is starting to get on our side!”
In the past world, Koutarou and Clan had gotten knowledge on the castle of Sariachal when they were thinking up plans of how to attack Maxfern and his group. One of the suggestions back then was to infiltrate via the underground aqueduct. Koutarou and Clan thought of trying that, and as luck would have it, the scout craft that was sent underground found the underground aqueduct had continued to be used. This was a big chance.
“Kiriha-san, what do you think?”
“... The other side doesn't know that we have this information. Even if they use the future forecast, the chances of them seeing us infiltrating from the aqueduct should be less than it really is. However... we should still use some randomness”
Kiriha said and pulled out cards from her bosom, fanning them out and presented them to Koutarou who drew one of them. Since Green relied on her future forecast, having several plans and deciding on which one to go with by drawing a random card like this was the safest way.
“... Then it's decided. Let's go with the underground aqueduct”
Koutarou had drawn an old card with most of its past metallic luster gone and had a motif of Kabutonga. On its surface were the characters that Kiriha had scribbled when she was young. It was a special card only Koutarou and Kiriha understood. The bingo card when deciding on strategies was always this card.