You can actually just show up and still get the AED100 entry; AED400 is for immediate entry, meaning you don't have to wait.

The trick is to go at a time of day when there are not a lot of visitors -- weekday morning has worked best for me -- and buy a pre-booked entry for the next available time (you may have to wait 15-30 minutes, but you still get the cheaper entry).

Even on a hazy day, it's quite a view and the outdoor viewing deck is lovely. Well worth the hype and cost!

--Chris