In the counterfactual, an upper gun deck would allow the large ironclad to fire down onto single-deck ships. This threat could make armored decks attractive earlier. The comparison is with the turtleback decks of the protected cruisers later associated with Elswick. Earlier adoption is a possible design response to the two-decker fleet.
Notable Point · 41:33-41:59
Two gun decks could bring armored decks forward
Downward fire from the hypothetical two-decker fleet could encourage earlier armored decks of the kind later associated with protected cruisers.