{def caesar

 {def caesar.alphabet {A.split ABCDEFGHIJKLMNOPQRSTUVWXYZ}}

 {def caesar.delta
  {lambda {:s :i :a}
   {A.get {% {+ {A.in? {A.get :i :a} {caesar.alphabet}} 26 :s} 26}
          {caesar.alphabet}}}}

{def caesar.loop
  {lambda {:s :a :b :n :i}
   {if {> :i :n}
    then :b
    else {caesar.r :s
                   :a
                   {A.set! :i {caesar.delta :s :i :a} :b}
                   :n
                   {+ :i 1}} }}}

 {lambda {:shift :txt}
  {let { {:s :shift}
         {:t {S.replace [^A-Z] by in :txt}} }
    {A.join {caesar.loop :s
                         {A.split :t}
                         {A.new}
                         {- {W.length :t} 1}
                         0 }}}}}
-> caesar


{def text VENI, VIDI, VICI}
-> text
{S.map {lambda {:i} {br}:i: {caesar :i {text}}}
       {S.serie 0 26}}
->

0: VENIVIDIVICI
1: WFOJWJEJWJDJ
2: XGPKXKFKXKEK
3: YHQLYLGLYLFL
...
23: SBKFSFAFSFZF
24: TCLGTGBGTGAG
25: UDMHUHCHUHBH
26: VENIVIDIVICI

As a shorter alternative:

{def CAESAR
 {def CAESAR.rot
  {lambda {:shift :txt :alpha :i}
   {A.get {% {+ {A.in? {A.get :i :txt} :alpha} 26 :shift} 26}
          :alpha}}}
 {def CAESAR.loop
  {lambda {:shift :txt :alpha}
   {S.map {CAESAR.rot :shift :txt :alpha}
          {S.serie 0 {- {A.length :txt} 1}}} }}
 {lambda {:shift :txt}
  {S.replace \s by in
   {CAESAR.loop :shift
                {A.split {S.replace \s by in :txt}}
                {A.split ABCDEFGHIJKLMNOPQRSTUVWXYZ}} }}}
-> CAESAR

{CAESAR 1 VENI VIDI VICI}
-> WFOJWJEJWJDJ
{CAESAR 25 WFOJWJEJWJDJ}
-> VENIVIDIVICI