I've found that medieval era seems a bit short in terms of tech compared to the others.
Given the popular view of this era as "dark times where few technological progress were made" (though this wasn't really true), may I suggest to keep the same techs in the ere, but significantly raise their cost ? (like x2 or x3 minimum)
This would give a specific "tech stagnation" feel to the era, while increasing its duration so we have time to play with all those castles and siege units