Leonardo da Vinci experimented in this depiction with the idea of a small set of stairs leading to the entrance. There is also a smaller drawing on the left hand side which does not seem to have carried quite the same level of importance to the artist.
You will see a stamp added to this drawing, which would have come about many years later after Da Vinci had complete it initially in 1487-1490. All of this series of works have a similar date attributed to them, it can be very hard to be anymore accurate over work that was produced so long ago and in this medium there was also very little supporting documentation to help art historians learn more about them.
This pen and ink is slightly rougher than his other pieces in this series. Perhaps he spent less time on it and merely wanted to experiment with one or two variations from his other related drawings.
The stamp mentioned on this drawing would have been added by its present owner - that of the Parisen Bibliotheque de l'Institut de France. They hold a fine collection of Leonardo da Vinci drawings which spearhead their overall collection.
There are additional notes written by the artist at the foot of this particular drawing. It was a sign of the importance that he placed on others carrying out his designs in precisely the same manner that he had intended.