Never open a chest while your inventory is full! This is because you can only take the artifact the first time you touch the chest, so it won't be there if you return after having made space.
@Reifam Nice analysis on Guile. I want to add that his grapple moves seem to have longer reach than most. His high forward momentum also makes these easier to pull off. But his aerial kicks feel clumsy to me and even his flashkick is quite flawed as it's easy to kick right through it.
Brilliant on lv20:
Destroy the top left totem early on for the crypt. Its freezing and scaring is very helpful against Smugglers. Focus on the lighthouses be4 focussing on boats, but keep a terror spell ready in case a diver reaches your gems.
Brilliant on 21 (final):
Keep the boat alive until the last wave as the pirates from it provide extra cash. Make sure the water exit of each cave is protected as the boss will send divers there. Save some Terrors for the ninja wave and any smugglers that may come close, save purple scrolls and/or meteors for Iron Soldiers as they can't be scared.
Good luck
Great game. Very instructive and very well integrated into an RPG/upgrade system. Can't wait for the sequel as the material was still a bit easy for someone with experience in programming languages :-)
I noticed some inconsistency in the Tip Calculator Part 4: Variables in the answer have to be used in opposite order of their appearance in both your code and the assignment. Will lead to unnecessary loss of health and/or need to use Hint.