A) I ignore protective leaders like the plague
B) Engineering requires a Great Engineer bulb. Great engineer specialist points are hard to come by (Unless I'm mistaken, I'm no master of the engineering bulb)
C) Castles suck hard. Unless you're Spain. Their trade route bonus is obsoleted by economics (weird choice I never understood, why not gunpowder or rifling) which isn't really an easy tech to skip and is really close on the tech tree. Of course I suppose that doesn't really matter as you are as I understand speaking of a really focused beeline, then why even bring castles up? Why waste time with those buildings, when instead you should be whipping out your Xbows and trebs? By the time they come up, why do you even need the defense?
D) Cavalry are not bad at city taking, they're actually really good. First of all, they often start with two promos thanks to barracks plus stable, they can start of with 3 if you run both experience civics and settle a GG in the city. So a fifteen strength unit with combat 2 = 18 strength, good odds on most medieval defenders. And effectual enough that it isn't too expensive to bring a bunch of them instead of siege. When they win they also gain lots of experience, so eventually you'll get a bunch of toughened veterans. And lots of great generals. Settle some, turn others into great medics and super strength city crackers, whatever you'd like. Trebuchets are good, but they lower exp gain, which is only necessary when you use units not strong enough to crack cities by themselves.