Keltham has in fact requested most of the things except an Alarm scroll and an Invisibility Purge scroll to be put back in the holding bag, so he can climb up once, empty out the bag, send the bag back down, instead of ferrying stuff up piece by piece.
Once he's up there and unloaded, Keltham casts Dancing Lights, for better illumination than the floor window.
He casts the Alarm spell on the space, also from scroll. (Keltham has spent a noticeable chunk of his salary on practicing with practice scrolls; it's a huge power amplifier if you've got money, and he expects to have money.)
Keltham considers, only then, if there's more he can do to protect the space...
And then starts to Prestidigitate the air around him, forming fragile shapes from it. (As ordinary wizards may also do, even not knowing what they're doing, just binding air molecules together to form a very fragile solid.)
By this means Keltham lays down a screen over the opening to the ground below, fine enough that it will let through air, and very little else.
He casts Invisibility Purge, then, also from scroll.
And when nothing then appears -
Keltham carefully lies down, in a position he's going to keep for a while.
And Keltham fills the whole volume except himself with fragile strands; webs that would shatter, visibly, at a touch.
...the Conspiracy can probably still beat that, with a nation's resources, but he's hopefully made it a bit hard for them.
Keltham needs, very much, to lie down and rest for a time.