"I'll try a gentler approach." Nar-heru will carefully walk across the bridge. If anything happens to cause a fall, he will activate his ring and fall gently, attempting to avoid any spikes.
Buckthorn will also try to gingerly cross the bridge (acrobatics?). Does there appear to be a mechanism, gear, lever, etc. that might turn the field on/off that Buckthorn can see? (Dungeoneering).
You--still--see no such lever. You move deftly across the field, but not deftly enough, and find yourself falling through. Your acrobatics does reduce the damage.
Its a little harder then climbing, say, a brick wall (athletics check, DC mid 20s, fail by more then 4 will fall, move at half speed, so a full round and couple of checks to cover the distance).