The Mac version, unlike the PC versions referenced in most of these reviews, doesn't have difficulty levels or allow for more gem collection (to buy upgrades) when replaying earlier levels. I can't advance past level 6. So, I got/bought about 1/40th of the game PC users did.
Defend the fortress from trolls who have come to raid your lands! Build towers, earn money, collect crystals and upgrade your spells – the enemy shall not pass! Repel the aggressor!
Mac version does not have the same game play as the PC version of Royal Defense Invisible Threat. The PC version plays like the original Royal Defense, which allows for gem farming. The Mac version of RDIT does not. It's an important point to consider when purchasing because it makes the game extremely difficult; unplayable, for some..