Before the war declaration, Oslo created a stable stair over the outer wall. They chose an area that wasn't particularly guarded due to the terrain, with them thinking it'd be difficult to climb.
It was like the enemy totally did not consider the enemy could have high-level earth elementalist in their midst.
Of course, they couldn't be blamed too much for this. The outer wall was too wide and it was impossible to assign a human guard every few meters. The coverage of the outer wall of a typical level 3 village would be tens of kilometers long.
This area definitely wouldn't be a problem if they were battling other villages, but who told them to go against Altera.
Further, the guards were already reduced in numbers thanks to the Alteran's hunting outside. These people were tired due to the mobs and were already spread thin, it was inevitable for them to find loopholes.
