I had the impression - from the high, arched ceiling, the heavy wrought-iron light affair hanging from it, and the quiet, discreet service, which was great - that this was a very old establishment.
Anyway, I liked it a lot. Very good food. The dorado - a sort of fish - that I had on the head waiter's suggestion was PERFECT, and so was the half-bottle of wine I had with it (on his suggestion too).
Most of all I liked the quiet, restful atmosphere.
Many of the other people eating there looked as if they may well be teachers at the university, but if so it would obviously be different outside the academic year.
Prices maybe slightly higher than in other places, but still very reasonable.
(The position of O Trovador makes it particularly ideal for anyone visiting the old cathedral, Either tired after seeing the cathedral in the morning and not wanting to face more steep Coimbra streets quite yet, or arriving and discovering it's closed for a midday pause and you'll have to wait until it opens again in the afternoon, you can go across the street and sit down and rest for a while in O Trovador, and enjoy a very, very good Portuguese lunch.)