Being this is my first working program in Common Lisp I would be very
glad to receive comments/opinions/suggestions about the source code
and what should I improve in my CL coding style.

Source code is here:
http://domdefelice.com/projects/scopa/scopa.clisp

It is playable with (play).

(I also wrote a post about it with an example of game session [1]).

Thanks,
dom

  (case foo
    ('bar 1)
    ('baz 42)
    ('quux 99))

This actually reads as:

  (case foo
    ((quote bar) 1)
    ((quote baz) 42)
    ((quote quux) 99))

If FOO evaluates to the symbol QUOTE, the first clause will always be
selected.

SBCL gives a style warning, at least:

  STYLE-WARNING:
     Duplicate key QUOTE in CASE form, occurring in the first clause:
    ((QUOTE BAR) 1), and the second clause:
    ((QUOTE BAZ) 42).
  STYLE-WARNING:
     Duplicate key QUOTE in CASE form, occurring in the second clause:
    ((QUOTE BAZ) 42), and the third clause:
    ((QUOTE QUUX) 99).

On a tangent, this also means that for a key to match the symbol NIL,
you must use the list form:

  (case foo
    ((nil) 1)
    ...)

If you just use this:

  (case foo
    (nil 1)
    ...)

...it's considered a list of no keys, and will never match.

- the return value of DELETE(-IF) has to be used, it doesn't necessarily
  modify the given sequence, or it might remove the first element of the
  sequence,

- GAME-STEP calls itself and PLAY recursively, which isn't guaranteed to
  be optimized away by a tail-call optimization, so do an explicit loop
  in PLAY or on another outer layer,

and more stylistic things:

- add an option to quit the game (user input 0?) (and perhaps check
  for wrong input too ...),

- multiple SETF clauses can be written in one statement, e.g.

- PRINC and TERPRI together probably aren't as clear as a single FORMAT
  call; in any case, a single FORMAT call is easier to edit later on,

- you could use PRINT-OBJECT instead of implementing separate DESCRIBE-
  functions, e.g. specialize it on the structs GAME, PLAYER, CARD, then
  simply print the object on the output stream to describe them,

- since you already use formatstrings quite often, you could also
  rewrite DESCRIBE-OPTION using a conditional as:

- is there another reason to use a list in HAND-SCORES-ASSIGMENT other
  than the COUNT calls in UPDATE-SCORES?  I'd use a structure here too.

- FROM-DECK and INTO-DECK could be called with unquoted symbols
  (like (from-deck hand ...)) if you move the quote to the macro
  definition, e.g. write them as