I survived 326 seconds. Was hoping to beat your score, alas I couldn't quite hack it.
The core gameplay loop is solid enough, though I was a little confused by "duration" vs "timer". I eventually realised the timer is acting as a sort of currency since it was ticking up? I'm still maybe a little confused about the relation ship of the timer to the rest of the game.
I do think it could be better demonstrated to the player how differentiated the units are. It seemed like some of them could shoot further vs have more health vs "tick" their damage at different rates. But that may have been vibes that aren't real - I assume there's a difference since they cost different amounts.
I think it also would have been fun if you utilised the water in the middle - there's boats but they just go on the roads. It would have been interesting to limit those to just water or air units because there's a bit of strategy on making sure you can afford those units to keep that lane safe vs the others.