They did, however, take until August 1855 to build the
900-meter pontoon bridge that enabled them to retreat to the north side when the south was taken. That's why I found your argument that "The bulk of the Russian forces would march to the Northern Side" so surprising, especially when coupled with the fact that the original plan was always to attack the North side first.
Perhaps I was not clear enough but you are definitely not paying attention to what is written. "The bulk of the Russian forces" was
Menshikov's army, which marched away from Sevastopol. The original allied plan did not and could not anticipate this move (which is rather clearly stated in the book you are quoted). This army did not have to cross the Big Bay to get to the North by the reasons obvious: it was not in the city and had a complete operational freedom. The allies did not know its size and (as, again, is stated in your book) tended to overestimate it.
Then, if you are referencing some events, it is helpful to be precise about the specific time frame. Just as it did not take a year for the Russians to build a defensive perimeter around Sevastopol, it did not take them " until August 1855" (which implies that they started in 1854) to build a bridge. Its construction started on July 14 of 1855 and the bridge was completed on August 15. In other words, construction took a single month. Not that this is relevant in a view of what I wrote about the main Russian army.\
This new suggestion that the Northern garrison will simply pile into ships and evacuate to the South is pretty much just a Russophile fantasy: if it had been possible for them to do so in the reverse direction, they wouldn't have bothered building the bridge.
Do yourself a favor and read something besides a single book. By the time the bridge was built the Russian fleet was on a sea bottom while at the start of the siege it was there and mostly intact except for the ships sunk at the entry of the Big Bay to prevent the allied fleet from entering it. And, BTW, keep the things alike "Russophile fantasy" to yourself because the whole perception can be labeled as "Anglophile" or "Francophile" fantasy based upon the assumption that the Allies had a perfect knowledge of the situation including all movements of the Russian troops, logistics, etc. The source of your quotations clearly indicates that this was a much bigger problem for the allies than for their opponents. So we are either assuming a reasonably OTL level of knowledge and the way of thinking on both sides or we are in a complete fantasy land.
I much prefer the man on the scene's opinion - particularly as you just have to look at the map of the city which you've posted in this thread multiple times to envisage the difference between shooting at the harbour from out at sea and from across the bay, which lets you fire directly into the heart of the harbour. If the fortifications on the north side weren't necessary to defend against from troops landing to the north and bombarding the harbour, one wonders why the Russians built them at all.
If you did look at the map, you must notice 3 big fortified batteries inside the harbor looking at the North side. Over 200 guns . Plus guns of the ships.
I doubt it, because Todleben states "
there were only 172 guns, all under 24-pounders, in the Forts" in the period we're considering. The Russians mounted more over time, of course, but the quicker the Allies move on Sebastopol the less time they have to mount guns.
Which "forts" had he been talking about in this specific case?
And the allies would need a time to put their own batteries in place under the enemy's fire.
Because, by the start of October 1854, the Russians had 32,000 troops in the Sebastopol garrison (closer to 50,000 by the end of the siege) and 341 guns on the south side alone. Immediately after the battle of Alma, they have less than 12,000 men and 29 guns on the mile-long northern front and 5,000 with 145 guns on the five-mile long southern front. Clearly, it's irrelevant whether the Allies bombard the harbour or move round to the South once they've taken the North - once that Northern garrison is neutralised, the rest of the city can't put up any meaningful resistance.
If bombardment across the bay is "irrelevant", why bother with the North at all instead of marching fast to the South and breaking through the almost non-existing defense line? The allies knew that the Southern side is weakly defended on the land and marched there. True, they overestimate strength of the Northern defenses but they also could not know at the time of their maneuver that Menshikov withdrew the field army from the city (his march was happening simultaneously with the Allied march to the South). Immediately after Alma the field army was still in Sevastopol and only
after he left the garrison amounted to 18,000 so the whole Northern scenario implies possibility of almost extra 30,000 being still in the city.
It is quite clear from the source you are quoting that the allies did not consider it possible to march to the South while keeping a sizeable detachment on the North because this would be too risky (their opinion). Then, with the line of communications open, who would prevent Menshikov from reinforcing the South while the allies are dealing with the North? Actually, he did just that in the early October.
"Neutralization" of the garrison of the North is another tricky issue, which I already addressed. A part of it would be probably lost but the rest could be evacuated or even retreat marching along the coast. The fleet, which was still mostly intact, could provide some cover.
And on page 81 it says "
Sir John evidently did not know that the garrison was so weak, the works so slight, that there was a breach in them, and, above all, that the covering army... had marched away and left the garrison to its fate." However, while we're all well aware of why the Allies marched round Sebastopol and began regular approaches historically, it's completely irrelevant in a scenario in which the explicit POD is the prompt capture of Sebastopol - which Todleben's own first-hand evidence illustrates would be far easier than you tended to suggest.
The logic is faulty because you are implying that
all 12,000 Russian troops on the South would be exterminated by the snap of the fingers and then the allies will teleport themselves on the South not giving its defenders any time to do anything. BTW, Totleben did not necessarily had the whole numbers because when he mentioned artillery on the North, he talks only about the guns in the existing fortifications. Wouldn't the troops on the North have their own guns?
Edit. Taking into the account that most of the troops defending the North (including Kornilov) moved to the South practically as soon as the allied maneuver became known, it does not look like the bridge across the bay was needed for this action. The fundamental difference from the eventual move to the North was in the fact that in the first case we are talking just about the foot soldiers with some artillery pieces and no heavy train and in the second we are talking about much greater numbers with a lot of extra things to evacuate: hospitals, a lot of artillery, supplies, non-combatants, etc.
I suspect that a big part of the troops in the first case had been transferred by the boats and small vessels, plenty of which had been available at that time.
Totleben, judging by the parts that I read, was talking about a possibility to destroy the fleet and arsenal by bombardment across the bay (but I did not find a direct quote, author just references to his opinion) , not about
capture of the city, which could be done only from the South. And, again, "
left the garrison to its fate" is author's statement (not Totleben's), which is not true. Look at your own numbers: initially, the garrison amounted to 17-18,000 but by the early October it was over 30,000. Why? Because Menshikov sent reinforcements and, by removing his army from the city, hold communications line open and could even make some diversions (like one at Balaklava) and could wait for the reinforcements, which kept arriving.