"Searchlights" is a side quest available in Dying Light.
Apparently Aflie discovered another way to protect the Tower: the Spotlights could help fight off the Infected's night attacks. [So that's what the things over the doors are for!] Problem is, the only bulbs strong enough to make this work are used to illuminate the Infamy Bridge. The tallest one in the city.
Location: Tower (Alfie)
Rewards: 5,000 Survivor XP, Electricutter Blueprint
The player must travel from Brecken's Tower in the Slums to the Bridge at the East side of the map at nighttime. Upon arriving at the base of the Bridge Towers, the player must climb up both of the towers to reach the struts across and retrieve the working bulbs from the spotlights located across the struts. There are two struts on two different levels, so the player will need to either climb up to the upper level for the second set of bulbs, or descend to the lower level for the second set.
Rather than risk being spotted during the night, The player should try to spend most of the day completing other tasks, then arrive at the base of the Bride towers near dusk and begin climbing. This eliminates any possibility of the player being chased by Volatiles and becoming confused during the mission.