Me and my girlfriend booked tickets at Portal Agency for the daily evening walking tour, lasting about two hours.
Our guide, an older man with beard, proved to be extremely knowledgeable about Trogir and Croatia but also about art, history and philosophy in general.
His explanations were somewhat chaotic though, sometimes meandering off topic. The information was also at times too in depth for laymen, requiring a lot of background.
Personally, I found the tour very interesting in spite of sometimes loosing track. I noticed however that fellow tourists lost interest rather quickly for abovementioned reasons.
Therefore I only recommend this tour to people with a passion for art and history.