oh how I wish my heart upgrade only cost 24k.. my current one (24h in, 24h to go) cost 144k.. quite depressing when you think about it, got to max xp for level 24, went to level, "oh no, I need to upgrade my hero trainer.." but then I discover that to do that, I need to upgrade my heart, but to do that, I had to upgrade my banks, so, a day spent levelling my gold and life storage, 2 days to upgrade the heart, then another day to upgrade my hero trainer, and then finally, I will be able to level my knight..
Of course, then, I start a whole new wave of upgrading my other stuff.. all of which will make it even easier for people to farm my castle.. game is very skewwed against the defender..