Thank you so much I will look into these!Have you played the Dungeon Warfare games? They are excellent with lots of cool traps and quite challenging.
The Fieldrunners games are also good for a more standard TD. As are Defend the Cake and OTTD: Over the Top Tower Defense.
If you are looking for something.a little different, I also recommend Dark Zone Defense. It's not a traditional TD. You are just one tower but your enemies are coming at you in the dark. Have to move your light around and listen and watch your warning light to know if something is getting too close.
Incredibly challenging with some originality with weapons and abilities. In fact, been meaning to mention it here for everyone. Believe it's also on sale right now for $.99.
Does the iOS app store not have a way of adding a game to a favourites list? I can't see an option anywhere!