There are several things wrong with the reconstruction seen above.
First, the location. Palace could not have been that close to the sea – under such conditions, sea will have broken against the foundations of the seaside wall, causing the entire southern wall of the palace, and both towers, to eventually collapse. Romans will have known this, though they will not have been aware of the fact that Adriatic coast is sinking as European continental plate is subsiding itself under African one. As such, palace will have been located some meters, or tens of meters, away from the sea shore during Diocletian’s time. Only the rising sea level will have brought it to the shoreline (indeed, nowadays the entrance to the cellars is below sea level).
Second, the greenery. There was no greenery on the walls, period. While Diocletian’s palace was indeed a retirement villa, it was also a military fortress designed to ensure emperor’s safety and peace in ever more chaotic world. As such, it had to be capable of defending itself.
Third, the towers. If one takes even a cursory look at the northeastern tower he will notice that the original construction rises only up to the level of the curtain wall. Anything above the level of the curtain wall is thus a later addition. Original towers will have been level with the wall, and also without a roof, but rather a terrace where artillery could be placed. This can be seen when looking at the fence atop the tower: there are three rows of stone blocks, yet windows at the level below start from the second row of blocks. Since there are no traces of windows at third row of blocks, there could not have been windows to begin with – and thus, no third level.