Ah yes, the Ancient Cistern. The top half reminds me a bit of the Spirit Temple, while the bottom is very much a cross between the Shadow Temple and whatever that area beneath the well in Ocarina of Time is. I don't think I've come across a Zelda dungeon before with quite such a light/dark split in it. The dark half (specifically the moment when you get the boss key) also has some wonderful foreshadowing for much, much later in the game.
I found it a reasonably straightforward temple on this playthrough, as while you do loop through the central room four or five times there's usually only one place to go to next. Plus it helped that I remembered or had written down bits and pieces of the route and puzzles from last time (including the temple's secret order). This is the only Zelda game where I've actually used a notebook as I've been playing through - normally there aren't enough different details to need one. As well as important text and symbols from the temples, the other useful note from last time is the tech tree (this game actually has one!).
The Stalmaster mini-boss was something of a rude surprise when it decided to pull out two more swords part-way through the battle. That thing does quite a bit of damage with all four swords, though it's also rather amusing to parry its uber-attack and watch all its arms drop off! Parrying is pretty much the way to go, as it defends well against ordinary slices and is very good at blocking Skyward Strikes.
And at the top of the temple... it's Ghirahim again! Who seems most annoyed that I'm still wandering around, and so decides to sic some giant corrupted six-armed chakram-throwing Buddha-like statue on me, which Fi claims was designed to defend the Ancient Cistern. One does wonder why, if there's some giant intruder-splatting-automation, it's hidden away right at the top of the temple where most intruders won't run into it? And why does it wait until after you've whacked it a few times before it starts walking around and hexa-wielding swords?
Useful tip for the battle, by the way: in the first phase, if you keep your distance it'll always throw chakrams at you which isn't particularly useful for actually beating it. You have to close with it before it'll try a different attack, at which point you can then use your whip on it. Also, in the first phase once you've disabled two arms you can then directly attack the core with a Skyward Strike when it tries to hit you (rather than waiting until you've removed four arms). This is definitely one of those bosses that is massively easier second time round when you actually know what you're supposed to be doing to it.
Anyway, my reward for beating that was Farore's flame and a rather handy double-strength sword. And also the Triforce of Courage, with a wonderful "WTF?" expression on Link's face. I don't think any of the other Links were quite so surprised to receive part of the Triforce.
Enough musing. Onwards and upwards to the Isle of Songs!