I want to reproduce the following figure.

[![enter image description here](https://i.stack.imgur.com/CT1g5.png)](https://i.stack.imgur.com/CT1g5.png)

And got the following figure with the code given below:

[![enter image description here](https://i.stack.imgur.com/qGYy6.png)](https://i.stack.imgur.com/qGYy6.png)

```
\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{arrows, shapes, trees, positioning}

\begin{document}

\begin{tikzpicture}[
node distance = 1cm
, level distance=1.15cm
,  every node/.style = {
shape=rectangle
%                                           ,  rounded corners
, draw=blue!50
, inner sep=1mm
, outer sep=0mm
,  minimum size=2.5cm
, minimum height=8mm
, align=center
, anchor=north
}
,  Trt/.style = {
shape=rectangle
,  rounded corners
, draw=black!90
,  top color=blue!10
,  bottom color=blue!30
, inner sep=1mm
, outer sep=0mm
,  minimum size=2.5cm
, minimum height=8mm
, align=center
, anchor=north
}
,  Th/.style = {
shape=rectangle
,  rounded corners
, draw=black!90
, inner sep=1mm
, outer sep=0mm
,  minimum size=2.5cm
, minimum height=8mm
, align=center
, anchor=north
}
,  Control/.style = {
shape=rectangle
,  rounded corners
, draw=green!90
,  top color=green!10
,  bottom color=green!30
, inner sep=1mm
, outer sep=0mm
,  minimum size=2.5cm
, minimum height=8mm
, align=center
, anchor=north
}
,  Comment/.style = {
shape=rectangle
,  draw=none
,  inner sep=0mm
,  outer sep=0mm
,  minimum height=5mm
,  align=right
}
,  level 1/.style = {
sibling distance=35mm
}
,  level 2/.style = {
sibling distance=20mm
}
,  level 3/.style = {
sibling distance=20mm
}
,  edge from parent fork down
,  edge from parent/.style = {
draw
,   semithick
, -latex
}
,
]

% Theraspist 1
\node[Trt]   (Trt)   {Treatment}
child {node[Th] (Th1) {Theraspist 1}
child {node (S1) {Subject 1}
child {node (S1Y)  { $\textrm{T}_{1}$,  $\textrm{T}_{2}$, $\textrm{T}_{3}$}}
}
}
child {node[Th] (Th2) {Theraspist 2}
child {node (S2) {Subject 2}
child {node (S2Y) {$\textrm{T}_{1}$,  $\textrm{T}_{2}$, $\textrm{T}_{3}$}}
}
};

%from bottom to top
\node[Comment] (L1)    [left=5.0mm of S1Y] {Level 1};
\node[Comment] (L2)    [left=5.5mm of S1] {Level 2};
\node[Comment] (L3)    [left=5.5mm of Th1] {Level 3};

% Theraspist 2
\node[Control]   (Control)   [right=45mm of Trt] {Control}
child {node (S3) {Subject 3}
child {node (S3Y)  { $\textrm{T}_{1}$,  $\textrm{T}_{2}$, $\textrm{T}_{3}$}}
}
child {node (S4) {Subject 4}
child {node (S4Y) {$\textrm{T}_{1}$,  $\textrm{T}_{2}$, $\textrm{T}_{3}$}}
};

\end{tikzpicture}
\end{document}

```

I wonder how to align the levels vertically as given in the first figure. Any help will be highly appreciated. Thanks