Sunset Spire is a Spire in Tyranny, activated at the Lethian's Crossing Runic Hall. Note that this article concerns only this specific Spire; for a general overview of Spire functionality, see Spires of Terratus.
Sunset Spire is the only Spire that can be unlocked without much hassle, if you skipped Lethian's Crossing and attacked other regions (eg. choosing Burning Library over the Crossing after securing Blade Grave for the Disfavored). In fact, Sunset Spire could very well be the first Spire you unlock in Act II, as the entrances to the runic hall and the Oldwalls are open (and you can find 2 charcoal rubbings to get a good idea of the runic hall pattern); note that the entrances will be locked once the Crossing is chosen as your next attack location.
Looming over Lethian’s Crossing since its creation, the Sunset Spire has always cast a shadow from the setting sun towards the market town at its feet. Despite being situated in the middle of a bustling settlement, very few have ventured inside. The Sunset Spire in Lethian's Crossing feels burdened by its age, as if a central linchpin somewhere in its core has weathered the centuries of oppressive weight, and longs to topple.
The surge of energy that courses through you, forming a link with Vendrien's Well, strengthens the very stones beneath your feet. The structural weakness you detected vanishes, replaced by confidence that could keep this tower standing for millennia.