The Spider boss is down and I've died once to a boss on a rooftop.
The Spider fight was slightly less interesting to me than the Shadows fight. I ended up killing all the adds while dodging ranged attacks. It took some tries to get the execution down, but it was pretty clear from the first couple of deaths what I needed to do.
One thing that bothered me was how quickly the Spider boss came on the heels of the Shadows fight. I spent some time pondering before jumping because I really didn't think From would put another boss fight so close to the last one. And the Shadows were a doozy, too.
Meanwhile the optional(?) castle zone is absolutely fantastic. The sounds are chilling and the architecture is amazing, both aesthetically and in terms of level flow. I'm almost sorry I've now found the quick route to the boss. I could have explored that level style for many hours and been pleased.
Bloodborne does a much better job than the Souls games of giving quick boss runs which, on one hand is a little sad because it makes the levels so much shorter (i.e. one spends much less time in them), but on the other is great for reducing the frustration of learning boss fights. Remember the Spider in DS2? Man, I hated that run to the boss.
You guys have a most dreaded Soulsborne boss run?
p.s. one of the reasons I decided to do this optional area now is that I started the next progression area and saw that it makes liberal use of a conceit that's been in every Souls game: the deeply embedded reanimator enemy. I have to say I'm a little tired of that mechanic -- all the mobs in the level getting resurrected until you find the necromancer responsible.