Hm...or maybe add a "crack" effect after the very first kick? And a message "the door cracks slightly". That would indicate that this is definitely destructible. Not sure if it would keep players kicking long enough though ;)

Maybe this should be explained in the tutorial dungeon?

I like ravenmore's suggestion.

I have added a message and I have added a tutorial hint. Let's try this first as adding a cracked door image would require yet another internal flag and more complex special handling. If newbie players in future test game still don't get it we can add the graphical details.