Construction beginning/end
After almost 60 years of climbing the wall, the explorers had created multiple bases from which to do their activities. These bases had generally been left alone by the denizens of the wall. This changed shortly after creating the base that is now called The Bulwark.
Normally creating walls and fortifications was only done after the ground around it had been secured and non-combatants began travelling to the base. Therefore they were unprepared as a seemingly endless amount of monsters of many different types all came to attack at the same time. Many of these creatures had never been observed to collaborate and several of them had even been observed to be strictly solitary. As such this attack came as a surprise and the base was lost. A few years later the base was retaken and walls erected. This time there never came a big attack. Nevertheless, the walls were still reinforced and built upon until they could hold a similar attack back.