The longer you play - even if you don't advance in the storyline - the harder the enemieswill get. Eventually even when buying all tech, tower and magical enhancements your enemies will grow stronger than you. Even the strongest free-to-play-towers on 3/25 won't help you as the strength counter doesn't seem to be capped. Probably this little "feature" shall drive you into buying stuff for real money...
The droprate for those 'epic' gems (like rubies, diamonds) could be drastically raised as the snowball effect of higher tiers is taking dozens of them for single items.
 "Prime World: Defenders"){kind=icon}
 "The Grinns Tale"){kind=icon}