The second floor was a huge dining hall. The light they saw from the outside was because half of the soldiers inside this fort were here enjoying drinks during their time off.
Luckily, no one was sitting near the stairs and the stairs had solid wood deck railings so Paytowin and Clearsky could duck and keep themselves out of sight while they made their way to the ground floor.
The ground floor was mostly empty. The few soldiers who stayed within the fort were mostly at the dining hall while the rest were on the walls outside, so they had no problem sneaking. Still, they searched carefully. All the while avoiding the front entrance because there was likely a guard there.