For seven long days, Erik and June kept watch over the fortress and its surrounding area.
Erik used his Plant Master power to make a simple shelter, which did two things: it hid them from the enemy, and gave them a good view of the surroundings.
Erik used his Plant Master power to build a hidden shelter high in an enormous tree. The shelter was perfect for watching the fortress without being seen.
Erik and June easily climbed it despite the absence of a ladder, since the tree had many natural spots to hold on to, which made climbing easier.
They didn't build a ladder because it might have been easy for Hin's guards to spot. Instead, they used their own climbing skills to reach and take care of their secret lookout spot.