Could someone check the structure and whether I'm using overlay/align correctly?

(require 2htdp/image)

(require 2htdp/universe)

(define (box-letter letter-size target-letter target-letter-color fill-color outline-color)
  (overlay/align "center" "center"
                 (cond
                   [(and(number? letter-size)(string? fill-color))
                    (square letter-size letter-size "solid" outline-color)])
                 (overlay/align 
                  (cond
                    [(and(string? target-letter)(string? target-letter-color))
                     (square letter-size "solid" fill-color)])
                  (cond
                    [(and(string? target-letter)(string? target-letter-color))
                     (text(number->string target-letter)letter-size target-letter-color target-letter)]))))