Eiro was laying on Lugo's back, looking into the sky as the stag was running next to the carriage pulled by the horses. The Demon was casually manipulating the liquid of the Ace of Cups to hide the presence of the monsters inside of the carriage as they approached the first gate into the town, the one closest to Eiro's manor.
And here, this would get a bit tougher, but it should be possible to let the monsters sneak by while Eiro spoke to the guards and distracted them a bit. Basically, he made sure to strengthen his aura in a specific way to attract the attention of the two guards as much as he could, doing the opposite of what the Ace of Cups did. Controlling that power to hide the monsters and this aura-manipulation at the same time was really a bit tougher than Eiro thought, but he was able to figure out how to do this in a stable manner pretty soon.