thanks for playing :) It's not a bug, its a feature, or rather a temptation! The upgrades stack up over time and the early levels require you to focus on building your army of turrets before you start upgrading them. Its pretty much designed so that at the end of the first round you have enough cash for a range upgrade - generally the worst kind of upgrade you can get in a tower defence game - and if you buy it that early you will lose the round. Rather if you save up and get a speed upgrade (the best of the 3) the latter rounds get a bit easier. It COULD be a bit more forgiving I guess! Thanks again for playing and rating :) I'm glad you had a bit of fun!