"coiner" - one who counterfeits coins. Not something we hear of much today, but in earlier times was a reasonably familiar sort of criminal.
"box-room" - a room in a house (usually a small room) which is just used for storing things, usually things that are not wanted very often. The collocation "box-room attic" is unusual, because these two words mean nearly the same thing. (Not quite, because a box-room could be anywhere in a house, not just under the roof).
"cistern": yes, it is still quite common for British houses to have a water-tank in the roof space - sometimes two (hot and cold). This was partly to provide a head of pressure for taps and showers, and partly to guard against interruptions in supply (and in the case of hot tanks, to provide a reservoir of heated water for when you needed it quickly, as in a bath).
There is not a literal tunnel, it is saying that between the tank and the sloping roof there is a long and narrow dark place (dark because it is behind the cistern).
"of course" is a parenthetical remark meaning "as you already know", or "as is obvious". It is much more common in speech than in writing, but of course this writing is meant to suggest somebody is speaking, and telling a story.
"For" says that the sentence is a reason or explanation of what precedes. So the meaning of "For of course he was thinking .. " is something like "(He was excited) because, as you probably realise, he was thinking ... "
From pole to pole refers to the North Pole and South Pole of a planet; it means all over the world.
In Wikipedia's article on Invictus, it has a section on its meaning:
Out of the night that covers me,
Black as the pit from pole to pole,
I thank whatever gods may be
For my unconquerable soul.
The first stanza depicts the speaker at night, in reflection. The poles referenced in the second line, the North and South poles, frame the entire world in a darkness, which is like that of a pit (not simply a hole; a place of incarceration, death though also alternatively like a Orchestra Pit).
Best Answer
It means they slept quite soundly (that is, quite deeply). Note, an entry in Willis's Current Notes, on page 48 of the June, 1857 issue, makes the following claim about “sleeps like a Top” or “as sound as a Top”:
Edit: Another source (Varieties of Literature: Being, Principally, Selections from the Portfolio of the Late John Brady, Esq., 1826) on page 14, quoting from Gentlemen's Magazine, 1793, gives a similarly-worded etymology for the phrase, suggesting it might be the source for the 1857 entry. Gentlemen's Magazine also commented: