Tree Grafting

Many smaller Eideli settlements utilize walls made of selectively grown trees instead of artificial materials. To do so ordinarily would take lifetimes- in the early days, it very well may have. However, during the time of the empire, such defenses were seen to by alchemists, whether local or traveling.
Tree grafting isn't just about sticking trees together, though a significant part of it is being able to do so much more quickly. It's also about being able to push a plant to grow thicker branches.
There's an element of planning and theoretical knowledge- knowing how to make one's work last, and to have the tree's natural processes work in favor of the build instead of deteriorating it.

Bloodwork

Tree grafting is quite well-known to be improved by the addition of blood rituals. However, it's illegal to do so without an alchemist's license. That being said, much of a tree grafter's work will be on small settlements- areas with little to no law enforcement due to low population. Unlicensed blood-users are often not reported to authorities, as they're generally much cheaper. Knowingly hiring an unsanctioned alchemist to perform blood magic carries penalties of similar severity to actually doing said blood magic, so there's also an element of mutually assured destruction.
This unspoken agreement does lead to tragedy, on occasion. As the unlicensed alchemist's education is generally incomplete, they may be unaware of certain nuances to rituals. A common mistake is the fact that one has to mask the scent of the blood that they're using, lest they attract the very predators they were trying to defend against in the first place. Sometimes, nothing that could pick up the scent will be in the area, and the settlement may assume that the job has been done right for months. Yet the scent may linger for years due to how it's integrated into the growing trees, and it becomes all the more likely to attract something that the walls can't stop.
Even if the walls hold up, the predators attracted still pose a threat (and are generally much worse hunting than the sorts of things not drawn to the scent of blood). Fumbling the scent-masking process can actually amplify it, further compounding the dangers of incomplete knowledge.