The One-Hundred and Thirty-Fifth Floor is the highest known Floor of the Tower. The Guardian is said to be extremely hostile and nobody has been able defeat it or pass its test yet, which is why the current "top" of the Tower is the 134th Floor. 
Tower inhabitants believe that Zahard will conquer the 135th Floor and continue scaling the Tower the day he's on the move again. 
It is also rumoured that when the current Lord of the Tower (Note: There are 3 Lords, they take turns in ruling the Tower every 1000 years) steps down and the next one starts ruling, then the next Floor will finally be conquered. It's because Molic One P. GR is a conservative ruler and it's not his priority to conquer another Floor. 
High Rankers earn the right to go up to the 135th Floor. But for now, not a single High Ranker has been known to have ascended to the next Floor.
To reach the 135th Floor and above, one needs the Key of the Tower, currently fragmented into the 13 Month Series and several crimson rings bearing the crest of Zahard, preventing anyone to reach the higher floors of the Tower.