Temp Tower Setup Troubles -- Slic3r (How do you format G-Code {if} statements?)
So I think I figured this one out.  I was looking at it a bit wrong.  And here's my code:
; Note to self:  layer_z = [layer_z]
; T tower floor 1
{if [layer_z]==1.6} M104 S225 T0
; T tower floor 2
{if [layer_z]==11.6} M104 S220 T0
; T tower floor 3
{if [layer_z]==21.6} M104 S215 T0
; T tower floor 4
{if [layer_z]==31.6} M104 S210 T0
; T tower floor 5
{if [layer_z]==41.6} M104 S205 T0
; T tower floor 6
{if [layer_z]==51.6} M104 S200 T0
; T tower floor 7
{if [layer_z]==61.6} M104 S195 T0
; T tower floor 8
{if [layer_z]==71.6} M104 S190 T0
; T tower floor 9
{if [layer_z]==81.6} M104 S185 T0
; T tower floor 10
{if [layer_z]==91.6} M104 S180 T0

The key to understanding what is going on is the ; Note to self:  layer_z = [layer_z] line.  The whole block of code above gets repeated for every layer of the print in the G-code, but it gets evaluated.  And when I search for &quot;layer z =&quot; and scan thru the code.. I eventually see this...
; Note to self:  layer_z = 1.6
; T tower floor 1
 M104 S225 T0                  &lt;---- Yowza. Nice. 
; T tower floor 2
; T tower floor 3
; T tower floor 4
; T tower floor 5
; T tower floor 6
; T tower floor 7
; T tower floor 8
; T tower floor 9
; T tower floor 10

and then again:
; Note to self:  layer_z = 11.6
; T tower floor 1
; T tower floor 2
 M104 S220 T0           &lt;---- Yes!
; T tower floor 3
...

All good. You have to have {} and [] within the code statements...
Addendum:  So it turns out things are not quite perfect.  I'm noticing at the upper floors, where things should be cooler, the temperature isn't changing.  Its locked at 215C.  I see this in the code:
; Note to self:  layer_z = 31.5999999999999      &lt;--- not sure how that happened.  
; T tower floor 1
; T tower floor 2
; T tower floor 3
; T tower floor 4
; T tower floor 5
; T tower floor 6
; T tower floor 7
; T tower floor 8
; T tower floor 9
; T tower floor 10

31.6 would have triggered a temperature change.  31.5999999999999 won't do it. I need to modify the {if... == } to a complex test between two values...   Just wanted to share that observation here.
I do see from this reference that

Expressions may be chained for an implicit AND:
{if [layer_num] == 10}{if [temperature_1] != 210}M104 S210

And this seems to fix it:
; Note to self:  layer_z = [layer_z]
; T tower floor 1
{if [layer_z]&lt;=1.65}{if [layer_z]&gt;1.55} M104 S225 T0
; T tower floor 2
{if [layer_z]&lt;=11.65}{if [layer_z]&gt;11.55} M104 S220 T0
; T tower floor 3
{if [layer_z]&lt;=21.65}{if [layer_z]&gt;21.55} M104 S215 T0
; T tower floor 4
{if [layer_z]&lt;=31.65}{if [layer_z]&gt;31.55} M104 S210 T0
; T tower floor 5
{if [layer_z]&lt;=41.65}{if [layer_z]&gt;41.55} M104 S205 T0
; T tower floor 6
{if [layer_z]&lt;=51.65}{if [layer_z]&gt;51.55} M104 S200 T0
; T tower floor 7
{if [layer_z]&lt;=61.65}{if [layer_z]&gt;61.55} M104 S195 T0
; T tower floor 8
{if [layer_z]&lt;=71.65}{if [layer_z]&gt;71.55} M104 S190 T0
; T tower floor 9
{if [layer_z]&lt;=81.65}{if [layer_z]&gt;81.55} M104 S185 T0
; T tower floor 10
{if [layer_z]&lt;=91.65}{if [layer_z]&gt;91.55} M104 S180 T0