At Portland State University’s crescent-shaped library, students can normally peer through floor-to-ceiling windows at the leafy green spaces below. But the library, under occupation since Monday by pro-Palestinian protesters, now has been turned into a makeshift fortress.

Chairs are piled around the windows. Wood pallets are stacked next to the doors as fortifications. Next to what is normally a circulation desk, students have established a medic center, preparing for what might happen if…