Hi Mark - this is a good question, but not if you realize how the citadel was constructed, and, understand the fact the compact prevented evolution of major explosives.
First, the upper cavern was HUGE, and deep, when flooded - the shipyard within probably could accommodate vessels drawing 15 feet or more, so the depth of the upper cavern, when filled, would be 18 feet or more.
The shaft leading down to the lower lock exit would be huge, too - as the water raising and lowering the ships had to accommodate hulls the size of Evenstar.
Therefore, the lowergate doors were not readily stormed by ram - the weight of the water pressure would jam them, and any breach would release a massive flood of water into a terribly narrow channel - effectively suicide. IF you stormed the doors when the lock was empty - now you had a sheer loch (picture a vertical rock shaft, probably three stories or more in height - and a second set of watertight, pressured by water, gates at the top…you'd have a nasty mechanical disadvantage to ram them, narrow space below, massive gates overhead, three stories, and again, a suicidal flood of water out of the cavern system if you breached - your crew would all drown.
The narrow cut, leading into the lock system was gated, and had guarding keeps that could create murderous crossfire, also, fire could be brought to bear from the citadel above.
Weak explosives (like black powder, or flour) cannot work in open spaces - they don't have enough ooomph. Research into our earth past shows to blow up a bridge with black powder - in the open - took TONS of the stuff…what makes it concussive is enclosed spaces. Flour, the same…it's an incendiary type explosive.
Therefore, to be effective, the attack has to happen INSIDE the loch shaft (at the outer side of the paravian defenses, which only protected the UPPER CITADEL). And INSIDE a tightly sealed hull - which Evenstar's hulk provided. Koriathain literally twisted the crew to their whim - so expect the flour was dispersed and lit off precisely so as to create as much damage as possible.
The explosion cracked the gates, the top one shut tight, the lower one, not yet open as the sluice was lowering the last bit of water…with the shaft sealed, all that had to happen, now, was a MINOR breach, top and bottom - with no flood of water from the emptied loch left to sweep out the cut, and the top gate's water let down, trammeled by wreckage, the swell through the channel would not have been as horrifically devastating, - AND the citadel at that moment was grossly undermanned (with the seagate under heavy attack by ship mounted siege towers) At night, in the noise and mess, the cut to the shipyard would have been, in that instant, vulnerable to a concerted assault. No one topside expected the area to be vulnerable, and communication in the citadel would have been slowed in the confusion.
The very narrow entry to the sewers - could have been shut at the shipyard end - but - the water outflowing, and only Cattrick and Fionn Areth, too little manpower to push the doors against the current…Further up the shaft, at the mid gate, was where the invasion was stopped. This gate was defensible, and the one topside, at the sewer entry - but the hydrology of the sewers themselves and the sluices and underground sea reservoirs engineered to flood the yard and lock - those cut rock channels were where the sappers could work…and were beginning to work…still faced by a daunting problem and tight defenses as the citadel held out above them. The water cisterns became the target, no actual breach of the upper defenses happened…as you saw.
At this point, I wish I'd done a more complete diagram map of Alestron's defenses…it's quite an interesting arrangement…too much to have described on so much depth at that pitch of action, in the story. Something to think on should I ever get a crack at doing a Preferred Edition!