transparent glass was not really a thing in ancient Greece. That said with careful design you could make it work with just small openings into the room which are covered except for occasional observation.

The greeks and egyptians had transparent glass, even glass magnifying lenses. https://en.wikipedia.org/wiki/Burning_glass

"your bailiff comes, shows me his writ - I, standing thus, d'ye mark me, in the sun's stream, measuring my distance, guide my focus to a point upon his writ, and off it goes in fumo!"