When the Enterprise first encounters the Reliant and right before she attacks, Kirk yells, "Yellow alert" and Savick says something about defense screens activating and we see a display of something happening. In the next scene though, Khan is informed that the Enterprise hasn't raised her shields yet. If defense screens aren't the same thing as shields, then what the heck are they?
Yellow alert automatically involves various ship's functions going active, which weren't at normal cruising operations, as an indicator of heightened readiness -- while not at battle alert. Phasers are energized, torpedo systems go live, but no targeting or loading is taking place yet.
Defensively, shields go up automatically at red alert, but not yellow. The Captain would have to order that separately. The "defense fields" you saw being energized are, basically, overcharging the Structural Integrity Fields in critical portions of the ship -- most significantly for us, the bridge superstructure. Makes sense if it remains an exposed portion of the ship, rather than being embedded in the hull.
Another oddity, on the Reliant we a diagram on one of the monitors, presumably one of the ones showing a diagram/layout of the ship. But it doesn't anything like Reliant, instead, it looks suspiciously like the secondary hull of the Enterprise. What gives with that?
Easiest fudge for that is that the
Enterprise and
Reliant both have about the same internal volume. If you discount the big hangar bays the
Reliant has to each side of the impulse engines, the remaining space would look a good bit like a section view of the
Enterprise's secondary hull.
Another question that came up when I was watching was, how did the Miranda class survive to the TNG era and the refit Connie didn't? Every time the Reliant was hit on the back of the ship or on the roll bar, or on the nacelles, we see the bridge explode in a shower of sparks and debris. Seems like a serious design flaw to me and would make the Miranda not exactly a ship design that you would want to keep around for overly long. That's unless they fixed that design flaw sometime after TWoK and made the Miranda a great design.
The
Miranda made a good Light Cruiser/Heavy Frigate. Compact, large and redundant shuttle facilities, flexibility in loadout. A good low-mass workhorse for low-risk areas. Remember, the
Reliant hadn't been expecting to run into Khan. The
Saratoga hadn't expected to run into the Whalesong Probe. The
Lantree was being used as a supply ship with a small crew. Etc. They're not front-line starships, but good supplementary vessels.
The
Constitutions, on the other hand, were ships-of-the-line. Their relevance relied on staying as bleeding edge as possible for as long as possible. A lot of the tech that went into
Enterprise's 2270s refit was derived from stuff developed and in use on the
Miranda class already -- saucer detailing, bridge module, phaser banks, navigational deflectors, warp-powered impulse engines, torpedo systems, and so forth. It all kept the class cutting edge for over a decade until the
Excelsior class started to roll off the blocks. Besides the fact that, by then, the class was second-string, the Khitomer Accords probably required Starfleet to retire one or more of its combat-focused classes. The
Excelsior class was brand-new and was reconfigured into Starfleet's first Explorer (the
Enterprise-B), but the
Constitution class was a relic of a conflict they were trying to put behind them, and almost half a century old by that point.
The initial notion was to make Picard's old ship that class, but they decided against it. The
Constellation class it ended up being was in part because it matched Geordi's mouth movement enough to overdub the scripted "Constitution class" line. But we do know they're still around -- at least mothballed indefinitely. One of them, at least, was activated to try to stop the Borg from reaching Earth, and was destroyed at Wolf 359.
Lastly, something that I've wondered about for the longest time. Where are the lights that light up the Starfleet symbol on the side of the secondary hull
Forward ends of the warp nacelles. Two sets of spotlights -- one angled toward the pennants.
and Enterprise on the very back of the Enterprise below the shuttle bay doors
Trailing ends of the nacelles. It's an awkward angle, but do-able with tight-focus lights.
Does the Enterprise have little spot lights that follow her along like remoras on sharks to light her up in the those and other areas where we don't see a light source. I'd noticed for the first time last nigh that the Reliant has some of the same invisible/mystery lights lighting up her nacelles right where her hull number sits. There's also invisible spotlights lighting up the Starfleet emblem on her pylons as well. I think that by TNG Starfleet seems to have gotten rid of these invisible spotlights that fly alongside their ships
Ah, the
Reliant, now... *sigh* The registries painted on the aft ends of the nacelles is self-illuminated. But the pylon markings are problematic.