The Southern Jungle

Even before the Shattering, the heavily forested southern regions where Eidel was founded were rather geologically unstable.
Essentially, within the Eideli region are pockets of extreme stability- bedrock said to reach to the center of the world. Outside of these, the structural integrity of the land tends to be questionable at best. It was often the case that flora was all that held the land together, making it rather impractical to build on.
This resulted in the Eideli tendency to build upwards, creating towering citadels that overlooked the surrounding lands.

Magical Influence

It's not a purely physical phenomenon which makes building land impractical- much like how the spirit wilds have myriad kami intrinsic to their lands, Eidel had a monolithic patron deity who served a similar purpose. Mother nature is a very literal phenomenon here, and she isn't kind to those that attempt to enroach. Most land is in a state of flux, slightly shifting in response to ambient flows of magic, consequently creating a very unstable foundation to build upon.
Similarly, the forces which act on the shifting lands aren't purely physical: one cannot just compensate for them with engineering.
Some theorize that Eid's true blessing was that of land: the Eideli people being given right to (some) foundations upon which they could erect permanent settlements, allowing them to rise above the thousand years of nomadic tribes who roamed the region.
Plants tend to be far more resistant to these ambient forces, even improving the area's stability.
There's a lot of magical noise in the region, it having a relatively strong connection to Fire, Storms, Darkness, and Blood- very unusual, considering that most regions typically have a single statistically significant ambient link. The exact values of the ambient affinities shifts seemingly at random.

Hazards

The primary hazard is an instability of terrain: the ground regularly shifts with the strength of a minor earthquake. Most trees have extensive root networks to compensate for this. On occasion, fissures in the ground will open up.
Plants often rapidly grow in spurts- a result of ambient magical energy shifting in an area, concentrations changing. There are certain plants which will flower or bloom only when a current of magical energy passes by. Some may remain dormant entirely until certain elemental affinities grow.

• Ghost Cap

• Burst Creepers are extremely prolific, and as their name implies, grow quite violently. This can cause heavy damage reminiscent of frost weathering in solid structures, further discouraging large-scale construction.

Arboreal Seas

There are some regions that are seemingly bottomless- the trunks of gargantuan trees stretch endlessly downwards, as far as the eye can see, and then some. Fetterer's Pass is famous for being a 'bridge' of solid ground spanning across one such area.