Cyteen takes place in the Alliance Union universe, along with books like Hellburner, Down Below Station and Merchanter's Luck. The new Alliance Union book was called Alliance Rising and takes place a bit earlier in the timeline than Cyteen does, before the war mentioned in that book. I believe all her SF is connected, the last Forigner cycle connected itself to Alliance Union, something she intended all along but could not do originally for publishing reasons. Chanuar books are also a part of that universe as well, if only remotely. Faded Sun, Hestia, Hammerfall, and other books all connect remotley, some from distant time periods. Supposedly Morgaine, her first series, is the furthest out along the timeline.
Fortress is not my favorite either but I liked the last book in the series, Fortress of Ice? I have often jumped into her series right in the middle and gone back. First book I read of hers was the last Chanuar and it made me fall in love with her writing. Take a shot at Fortress of Ice maybe. She is good enough that it usually is not a problem.