I dug out my old copy of Noctropolis to give it a try, and.......
Here we go - create a folder on your C: drive (mine is called "games")
Open DOSBox - you'll see a z: prompt, so you typez: mount c c:\games
now insert the Noctropolis cd into your cd drive, and mount the drive (my drive is H:) and typez: mount d h:\ -t cdrom
change to your (DOSBox's) D: drive - z: D:
This will create a folder - NOCTROP on DOSBox's C: drive.
Change to the C: drive - D: C:
C: cd noctrop
and thenNOCTROP: darkrun
You will need to leave the CD in whilst you play.
Alas, I have the game running without sound - when I get time, I shall sort that out (unless someone else beats me to it!).
As a rule of thumb, when you have installed DOSBox it is worth opening the config file at C:\Program Files\DOSBox with Notepad and scrolling down to the very bottom, where it says [autoexec]
# Lines in this section will be run at startup.
underneath that, add mount c c:\games
mount d h:\ -t cdrom
and DOSBox will open with those drives already mounted. The DOSBox WIKI at http://www.dosbox.com/wiki/Main_Page
has all sorts of useful info to help you on your way!