...Also, a clever conqueror would just go around the stations and challenge the Cardassian mobile forces where they would enjoy no support from fortifications, then
move against the more heavily defended targets.
However, it does seem that Cardassian defenses are not to be sneered at. We never got the impression that the fortification technology used in "Tears of the Prophets" would be new to the Cardassian Union as such - it may be just that the beam weapons got a tad stronger, the torpedoes gained a bit more bang, from the new power arrangements. There seems to have been a quantum leap in Cardassian weapons strength between "The Wounded" (which probably features the same weakling weapons used in the old war with the Federation) and DS9 (where the pink beams have been replaced by amber ones that actually can hurt somebody - and perhaps again when the Dominion provided aid. But the actual physical elements of defense may not have changed much, considering that the starships certainly don't change much during these technology leaps.
An invasion thus supposedly involves large starship forces, such as those seen in "Tears of the Prophets", and not merely the sort of fifty-ship formations seen in "Way of the Warrior". The dialogue in this latter episode certainly allows for very large Klingon fleets, as the only numerical value we get are "over a hundred ships in the first wave already" and "at least twenty warships in the vicinity of the station".