You must use one turn to move from the Valley of Fire onto the Crown of Command and then you must wait until your next turn to begin casting the Command Spell. That's how my group's always done it, at least.
Then if a character is on the Crown of Command and a second character moves from Valley of Fire to Crown of Command, then the 2nd character has to wait one turn before attacking the first one? Do you really play like this, when you did it another way over the entire game?
When you land on a space you follow instructions for that space. Instructions for the Crown of Command are detailed on page 20 of the basic Rulebook and nowhere is said that you have to wait one turn before casting the Command Spell. On the contrary, it is said that a character who is alone on the Crown during his turn (not at the start of his turn) must cast the Command Spell.