JosEPh_II
TBS WarLord
Because with Bronze Working they get replaced with a newer unit and unless you rush for the Ship Building tech that is requirement for Tireme you most likely will have Bronze Working before.
So unless there is some way, I don't know about, to build replaced units I can't build them any more.
In the BUG Options screen never check Hide obsolete buildings/units selections. If you do this Triremes are accessible after Bronze Working. I think another says something about Hiding Replaced buildings/units. I never check that on either.
The only ones I check are Hide Untrainable and Unbuildable (they worded slightly different but when you look at that screen it's easy to tell what I'm trying to say)
Checking the Hide Obsolete has caused many a player problems just like you. So I suggest don't use that Option.
JosEPh