mjhjmtu

My question is: how to perform calculations in foreach variables.

Here is my code

documentclass[tikz,border=3.14mm]standalone
usetikzlibraryshapes
begindocument

begintikzpicture
%-> USEFUL DEFINITIONS
deftfs0.1 %... the fence width and height
deftth2 %... the tower height
defttl5*tfs %... the tower length
deftwh1 %... the wall height
deftwl20*tfs %... the wall length
defenvcolorgray!50 %... color for everything

%-> DRAW THE CASTLE
fill[envcolor]
%... tower
(0,0) rectangle(ttl,tth)
 ;
foreach x in 0,0.2,...,0.4
 fill[envcolor]
 (x,tth) rectangle(x+tfs,tth+tfs)
 ;
%... wall
fill[envcolor]
 (ttl,0) rectangle(twl,twh)
 ;
foreach x in 0.6,0.8,...,1.8
 fill[envcolor]
 (x,twh) rectangle(x+tfs,twh+tfs)
 ;
%... tower bricks
foreach y in 0,0.2,...,tth
 foreach x in 0,0.2,...,2
 draw[white]
 (x,y) rectangle(x+2*tfs,y+tfs)
 ;
foreach y in 0.1,0.3,...,tth
 foreach x in 0.1,0.3,...,2
 draw[white]
 (x,y) rectangle(x+2*tfs,y+tfs)
 ;
%... wall bricks
foreach y in 0,0.2,...,tth
 foreach x in 0,0.2,...,2
 draw[white]
 (x,y) rectangle(x+2*tfs,y+tfs)
 ;
foreach y in 0.1,0.3,...,tth
 foreach x in 0.1,0.3,...,2
 draw[white]
 (x,y) rectangle(x+2*tfs,y+tfs)
 ;
endtikzpicture
enddocument

But I would like to do something like below:

%... tower bricks
foreach y in 0,2*tfs,...,tth
 foreach x in 0,2*tfs,...,ttl-tfs
 draw[white]
 (x,y) rectangle(x+2*tfs,y+tfs)
 ;
foreach y in tfs,3*tfs,...,tth
 foreach x in tfs,3*tfs,...,ttl-tfs
 draw[white]
 (x,y) rectangle(x+2*tfs,y+tfs)
 ;
%... wall bricks
foreach y in 0,2*tfs,...,twh
 foreach x in ttl,ttl+2*tfs,...,twl-tfs
 draw[white]
 (x,y) rectangle(x+2*tfs,y+tfs)
 ;
foreach y in tfs,3*tfs,...,twh
 foreach x in ttl+tfs,ttl+3*tfs,...,twl-tfs
 draw[white]
 (x,y) rectangle(x+2*tfs,y+tfs)
 ;

I've simplified your code a bit. I think you're thinking too much like a brick-layer adding the mortar as you go along. I fill in the mortar at the end.

Here's the code:

documentclass[tikz,border=3.14mm]standalone
colorletmain wall colorgray!50
colorlettower wall colorblue
colorlettower fence colorgreen
colorletmain fence colorred
colorletmortar colormagenta

colorletmain wall colorgray!50
colorlettower wall colormain wall color
colorlettower fence colormain wall color
colorletmain fence colormain wall color
colorletmortar colorwhite
begindocument

begintikzpicture[%%
% x=(1mm,0),%% uncomment to get desired scale
% y=(0,1mm),%% uncomment to get desired scale
 ]
%-> USEFUL DEFINITIONS
deftfs1 %% the fence width and height 
deftth20 %% the tower height 
pgfmathsetmacrottl5*tfs %% the tower length 
deftwh10 %% the wall height 
pgfmathsetmacrotwl20*tfs %% the wall length 

%... tower
%% you used `fill` here which causes a seam to appear between
%% the main portion of the castle body and the tower. I've modified
%% the code in these next two lines to remove that effect.
%% The second draw prevents bricks from sticking out of the sides.
draw[tower wall color,fill] (0,0) rectangle (ttl,tth) ;
draw[mortar color] (ttl,tth-twh) -- (ttl,tth);

%% the fence along the top of the tower
foreach x in 0,2,...,4
 
 %% use draw instead of fill so that the brick doesn't seem to overlap the mortar
 draw[mortar color,fill=tower fence color] (x,tth) rectangle (x+tfs,tth+tfs) ;
 

%% you used `fill` here which causes a seam to appear between
%% the main portion of the castle body and the tower. I've modified
%% the code in these next two lines to remove that effect.
%% The second draw prevents bricks from sticking out of the sides.
draw[main wall color,fill] (ttl,0) rectangle(twl,twh) ;
draw[mortar color] (twl,0) -- (twl,twh);

%% the fence along the main body of the castle
foreach x in 6,8,...,18
 
 %% use draw instead of fill so that the brick doesn't seem to overlap the mortar
 draw[mortar color,fill=main fence color] (x,twh) rectangle(x+tfs,twh+tfs) ;
 

%% horizontal seams of mortar
foreach y in 0,1,...,20
 
 draw[mortar color] (0,y) -- (20,y);
 

%% create the vertical seams of mortar which are offset
%% differently for odd and even layers of mortar.
foreach x in 0,1,...,20
 
 foreach y in 0,1,...,20
 
 ifoddyrelax
 ifoddxrelax
 draw[mortar color] (x,y) -- ++(0,1);
 fi
 else
 ifoddxrelax
 else
 draw[mortar color] (x,y) -- ++(0,1);
 fi
 fi
 
 

endtikzpicture 
enddocument

There are a couple things to notice here:

Color coding to facilitate construction

I've color-coded the parts of the diagram. There are several reasons for this. First is that I want to be able to see what it is that I'm doing. This makes the different parts stand out. Only for the final finished product will I remove the color as I did above.

If you comment out the following lines, you can see this.

colorletmain wall colorgray!50
colorlettower wall colormain wall color
colorlettower fence colormain wall color
colorletmain fence colormain wall color
colorletmortar colorwhite

Then the castle will look as follows:

Also, by naming the colors for each part, you make it easier for yourself weeks or months later when you want to tweak the castle but don't quite remember what each part of the code does. You could put in comments; always a good thing. But, naming the colors this way also helps you see what is supposed to be happening in each part of the castle construction.

Scale

The next thing I did was the scale. Work on a scale that makes this easy for you to see what's going on. You can readjust the scale to something closer to what you want by uncommenting the lines:

% x=(1mm,0),%% uncomment to get desired scale
% y=(0,1mm),%% uncomment to get desired scale

which will produce

Also, just in terms of computations, I prefer to work with integers over decimal because then I have more control over the precision and need to worry much less about round-off errors and the like. If you're going to scale like this, though, you need to be aware of what exactly is being changed. So, you should probably read up on this in the TikZ manual. Any rigidly defined lengths are not going to be rescaled. There is also a scale key which you might find useful too.

Using relative coordinates

Instead of writing:

draw[mortar color,fill=main fence color] (x,twh) rectangle(x+tfs,twh+tfs) ;

You could write

draw[mortar color,fill=main fence color] (x,twh) rectangle ++(tfs,tfs) ;

This is a step toward using relative coordinates which can ultimately make your picture much more amendable to future modifications. Also, I believe the ++(<coordinate>) notation makes much clearer in the code what it is that you're doing. But that's probably just personal style on my part.