The embassy utilises apartments on the ground floor. The only floor below it is the basement. The embassy isn't the only tenant of the building and all floors are accessible to all by using the stairwell or lift.
If you read the news article here, you'll see that the police guarded the stairwells and lift in the early days to stop him making a dash up to the roof. http://www.dailymail.co.uk/news/article-2189749/Threadbare-room-inside-Ecuadorian-embassy-Julian-Assange-hiding.html