Archive.org. I read the whole thing, and my post on the overall story of the Babylon excavations is here.
But that still doesn't get us to the top; how did Koldewey know what the tops of the walls and towers looked like? So far as I can tell, he did not. Archaeologists can often guess at the height of walls from the amount of rubble, but at Babylon the upper layers had been mined for bricks for 2,000 years, so that would not be very reliable. And sometimes you can tell a lot about a building from particular cut stones or shaped bricks, but that does not seem to be the case at Babylon, where the bricks were all made to standard shapes and sizes.
So the reconstructed gate is 90% original about halfway up. Above that it may be largely imaginary, although it is built of original Babylon bricks. The wonderful sculpted animals are as real as anything you see in a museum. I come to the end of this exploration of Babylon feeling reassured more than worried; Koldewey and his crew did an amazing job, and the maps on which all our reconstructions of Babylon are based are as good as it gets. The tops of the buildings are a lot less certain than the bottoms, but that is just always the case in archaeology.