In addition to UtMs proposals, I have two more possible reasons:
1) Study the crystal. Hydlaa is as near to the center, and thus to the crystal, as one can possibly get without flying. Building a tower to study the crystal more easily seems plausible if we are only looking to eliminate surrounding light sources and possibly also surrounding magical effects that one would have in the streets, while at the same time not being in the wilderness (not in danger and possibly comfy) and in close range to the institutions supporting the research.
The tower isn't very high compared to the distance to the crystal (20km according to the geography book), so there's not much benefit gained from the additional proximity, though.
2) Warning fire / relay. The first level is the only one that has access to the stone labyrinths (via the bronze doors), so all danger must pass through it. One can also see from one corner of the level to the other, but that certainly depends on weather. It therefore makes sense that the fortresses that guard the bronze doors light an alarm fire when there is an attack, and the Hydlaa tower relays that alarm so not only all fortresses, but also all settlements are informed, even those with no direct line of sight to any given fortress.
Edit:
this and other developments as the cone turns.
Does that mean the Yliaki know that the planet they live in turns? Or is there some mechanism that turns the stalactite, like in The Cube?
