The Tower of Sheagil is Book 3 of The Emperor’s Foul Legacy.
What started as a kind of holiday, traveling through Relanor, takes an odd turn and then a sinister one. Odd because Falia and Olcish are still switching places. Sinister when Falia gets a message meant for Olcish from someone she thought was only a legend. Sheagil has unfinished business.
This is the kind of story that has appendices and a map.