Just Another Dungeon Game - Review/Tips Appreciated.

Works great!

You could do a few things to clean up the code (like using the PEP 8 style guide for Python coding), and shorten/combine some things:

1. Use a list comprehension to generate the map grid and return in one line:

   ### before ###
   def generate_map_grid(size):
     new_map = []
     for y in range(size):
       for x in range(size):
         new_map.append((y,x))
     return(new_map)
   
   ### after ###
   def generate_map_grid(size):
     return [(y,x) for y in range(size) for x in range(size)]
   

2. Shorten all the conditional checks that are comparing to booleans, by changing == False to use not before the variable, and removing == True:

   ### before ###
   if map_dict[location] == 'empty':
         print ("[ ]", end='')
       elif map_dict[location] == 'breadcrumb':
         print("[~]", end='')
       elif map_dict[location] == 'monster':
         if playing_game == False:
           print("[X]", end='')
         else:
           print("[ ]", end='')
       elif map_dict[location] == 'door':
         if playing_game == False:
           print("[@]", end='')
         else:
           print("[ ]", end='')
       elif map_dict[location] == 'player':
         print("[P]", end='')
   
   ### after ###
   if map_dict[location] == 'empty':
         print ("[ ]", end='')
       elif map_dict[location] == 'breadcrumb':
         print("[~]", end='')
       elif map_dict[location] == 'monster':
         if not playing_game:
           print("[X]", end='')
         else:
           print("[ ]", end='')
       elif map_dict[location] == 'door':
         if not playing_game:
           print("[@]", end='')
         else:
           print("[ ]", end='')
       elif map_dict[location] == 'player':
         print("[P]", end='')
   

   ### before ###
   if playing_game == False and not quit == True:
   
   ### after ###
   if not playing_game and not quit:
   

   ### before ###
   while playing_game == True:
   
   ### after ###
   while playing_game:
   

   ### before ###
   if moved == True:
         map_dict[old_player_loc] = 'breadcrumb'
         map_dict[player_loc] = 'player'
       else:
         render_map = False
   
       if player_loc == door_loc:
         playing_game = False
         map_dict[player_loc] = 'door' #to show location of door during final map render instead of P
         render_live_map()
         print("\nYOU FOUND THE DOOR! YOU WIN!")
       elif player_loc == monster_loc:
         playing_game = False
         map_dict[player_loc] = 'monster' #to show location of monster during final map render instead of P
         render_live_map()
         print("\nGAME OVER. THE MONSTER GOT YOU.")
       elif render_map == True:
         render_live_map()
   
   ### after ###
   if moved:
         map_dict[old_player_loc] = 'breadcrumb'
         map_dict[player_loc] = 'player'
       else:
         render_map = False
   
       if player_loc == door_loc:
         playing_game = False
         map_dict[player_loc] = 'door' #to show location of door during final map render instead of P
         render_live_map()
         print("\nYOU FOUND THE DOOR! YOU WIN!")
       elif player_loc == monster_loc:
         playing_game = False
         map_dict[player_loc] = 'monster' #to show location of monster during final map render instead of P
         render_live_map()
         print("\nGAME OVER. THE MONSTER GOT YOU.")
       elif render_map:
         render_live_map()
   

   If you don't like the look of that code, because you feel it makes it less clear, then maybe you could rename the variables to suit:

   ### from ###
   if moved:
         map_dict[old_player_loc] = 'breadcrumb'
         map_dict[player_loc] = 'player'
       else:
         render_map = False
   
   ### to ###
   if has_moved:
         map_dict[old_player_loc] = 'breadcrumb'
         map_dict[player_loc] = 'player'
       else:
         should_render_map = False
   

3. In your main game loop, you're using continue within the if/elif/else conditional statements. You don't need that, since it will exit the if/elif/else after it completes the matching block, and loop again once it reaches the end of the while block:

   ### before ###
   while playing_game:
   
     move = input("\n> ").upper()
     render_map = True
   
     if move == "QUIT":
       playing_game = False
       quit = True
       render_live_map()
       print("\nOK. You quit. Bye...")
     elif move == "HELP":
       show_help()
       continue
     elif move == "LEGEND":
       show_legend()
       continue
     elif move == "MAP":
       render_live_map()
       continue
     elif move == "L" or move == "R" or move == "U" or move == "D":
   
       player_loc, old_player_loc, moved = move_player(move, player_loc)
   
       if moved:
         map_dict[old_player_loc] = 'breadcrumb'
         map_dict[player_loc] = 'player'
       else:
         render_map = False
   
       if player_loc == door_loc:
         playing_game = False
         map_dict[player_loc] = 'door' #to show location of door during final map render instead of P
         render_live_map()
         print("\nYOU FOUND THE DOOR! YOU WIN!")
       elif player_loc == monster_loc:
         playing_game = False
         map_dict[player_loc] = 'monster' #to show location of monster during final map render instead of P
         render_live_map()
         print("\nGAME OVER. THE MONSTER GOT YOU.")
       elif render_map:
         render_live_map()
   
     else:
       print("\n*** Invalid selection. Type help to see commands.")
       continue
   
   ### after ###
   while playing_game:
   
     move = input("\n> ").upper()
     render_map = True
   
     if move == "QUIT":
       playing_game = False
       quit = True
       render_live_map()
       print("\nOK. You quit. Bye...")
     elif move == "HELP":
       show_help()
     elif move == "LEGEND":
       show_legend()
     elif move == "MAP":
       render_live_map()
     elif move == "L" or move == "R" or move == "U" or move == "D":
   
       player_loc, old_player_loc, moved = move_player(move, player_loc)
   
       if moved:
         map_dict[old_player_loc] = 'breadcrumb'
         map_dict[player_loc] = 'player'
       else:
         render_map = False
   
       if player_loc == door_loc:
         playing_game = False
         map_dict[player_loc] = 'door' #to show location of door during final map render instead of P
         render_live_map()
         print("\nYOU FOUND THE DOOR! YOU WIN!")
       elif player_loc == monster_loc:
         playing_game = False
         map_dict[player_loc] = 'monster' #to show location of monster during final map render instead of P
         render_live_map()
         print("\nGAME OVER. THE MONSTER GOT YOU.")
       elif render_map:
         render_live_map()
   
     else:
       print("\n*** Invalid selection. Type help to see commands.")
   

4. There's a few places where you're using print just before you use input. You could just combine the two and only use the input, to save another function call. If you're worried about the argument to input being too large, move the prompt message to a variable, which would also make it easier to change later:

   ### before ###
   print("\nARE YOU READY TO ENTER THE DUNGEON?\nType Y if yes, or anything else to quit.")
   if input("> ").upper() == 'Y':
   
   ### after ###
   if input("\nARE YOU READY TO ENTER THE DUNGEON?\nType Y if yes, or anything else to quit.\n> ").upper() == 'Y':
   
   ### or ###
   ready_message = "\nARE YOU READY TO ENTER THE DUNGEON?\nType Y if yes, or anything else to quit.\n> "
   if input(ready_message).upper() == 'Y':
   

5. If you're printing the same variable twice using the format function, you can use the index twice in the string you're formatting:

   ### before ###
   print("\n{}*{} map successfuly generated.".format(map_size,map_size))
   
   ### after ###
   print("\n{0}*{0} map successfully generated.".format(map_size))
   

6. It looks like you repeat a lot of code when setting up a new game. Maybe move that to a new function? It'd mean changing a whole lot of other stuff so that you're passing in parameters to your other functions for things like the map_grid and map_dict. This would be a much bigger change of course.

7. It seems a bit strange to me to have the grid as a list and then a dict to hold information about particular locations within that grid and what is in those places. Why not have either a nested list with the inner lists containing the coordinates and what is in that cell, or use a dict within each cell, with, say keys x and y for the coordinates and occupant or something to describe what is there. Then you would just always access the one list.

8. You could change to a class-based system, which I think is covered in another Python course.