The Nightspider is one of the more complicated spelljammers in the book. The areas depicted in rusty red on the left image are hatches that open to reveal the interior of the vessel. Both the battle deck, where the ballistae and mangonel are held, and the main deck are revealed.
I stuck as close as possible to the published diagram, but this one is a little complicated. I simplified the ship by spreading out the decks that overlap each other. It won’t match up exactly with the diagrams in the Astral Adventurer’s Guide, but it is very close.