I think if you've never written any sort of lower level code before it's somewhat more daunting. I did spend a little time trying to get this emulator to run but ran into too many issues and gave up -- it's either a problem with some optimizations no longer being valid and raising type errors by default now (e.g. specifying a type as string does not, like other languages, implicitly mean it's either string or null¹) or something about the sdl2 bindings has changed (it's not the most stable of library binds unfortunately and has some misleading docs/examples out there, and the with- macros are not good and best avoided).

The assembler project I linked on the other hand works, though as you say having multiple asdf files is annoying, and the dollhouse.lisp demo makes an unfortunate choice of setting its *path* to #.*compile-file-pathname* Here's what I did to run it without changing that line (which would be better), translate to your flavor of editor+slime for convenience:

    $ git clone https://github.com/ahefner/asm6502
    $ cd asm6502/
    $ sbcl
    * (asdf-here)
    ; my own function I have in ~/.sbclrc, it is just:
    ;(defun asdf-here ()
    ;  (push (uiop:getcwd) asdf:*central-registry*))
    * (asdf:load-system "ichr")
    T
    * (defpackage :dollhouse-demo)
    #<PACKAGE "DOLLHOUSE-DEMO">
    * (defvar dollhouse-demo::*path* "hacks/")
    DOLLHOUSE-DEMO::*PATH*
    * (load "hacks/dollhouse.lisp")
    ; ...output
    ;
    ;(:NUM-UNIQUE 458) 
    ;Empty space begins at F600
    ;Created "/tmp/dollhouse.nes"
    T
    * ^D
    $ fceux /tmp/dollhouse.nes 

¹Example of SBCL raising a type error:

    (let ((x 3))
      (declare (type integer x))
      x)
    ; -> 3
    
    (let ((x nil))
      (declare (type integer x))
      x)
    ; in: LET ((X NIL))
    ;     (LET ((X NIL))
    ;       (DECLARE (TYPE INTEGER X))
    ;       X)
    ; 
    ; caught WARNING:
    ;   Constant NIL conflicts with its asserted type INTEGER.
    ;   See also:
    ;     The SBCL Manual, Node "Handling of Types"
    ; 
    ; compilation unit finished
    ;   caught 1 WARNING condition
    
    debugger invoked on a SIMPLE-TYPE-ERROR in thread
    #<THREAD tid=9761 "main thread" RUNNING {1001750003}>:
      Value of NIL in
      (LET ((X NIL))
      (DECLARE (TYPE INTEGER X))
      X)
    
      is
        NIL,
      not a
        INTEGER.