I think the author was on just the other day, you could ask him. However, I am guessing it isn't any sort of generic engine, but instead something especially coded just for that game.
Oh fine I'll answer
Lol
So if you view the code, navigate to
DEF XROT
DEF YROT
DEF ZROT
DEF POINT3D
These 4 functions make up the 3D engine in the game
XROT will rotate an x,y,z point around the x axis
What POINT3D does is it takes an x,y,z position, and gives you the x,y[,z] position of where it goes on screen
So yes I made the 3D engine, but it's very very easy to make
If you know how to program at an intermediate level, you could make a program that manages multiple objects at once, storing each point as x,y,z
And when your ready to draw, you go through each point and use POINT3D to draw where it should be on the screen
Oh and you would use XROT,YROT,ZROT to rotate each point around the camera
If you just want to use a 3D engine and not get into all that stuff, then I highly recommend the P3D engine
P3D: [735YW3L4]
It may take some time to learn though
But the result looks very nice