Bloons Tower Defense is the series that perfected the genre's balance. Balloons follow a fixed path and you place monkey towers beside it to pop them, spending the money each pop earns on more towers and upgrades. Simple — until the balloons start arriving in layers, where popping a red reveals a blue, then a green, then something armoured that ignores half your defences.
That layered-balloon design is what makes it deeper than most tower defence: your tower mix has to handle not just quantity but the specific properties of later rounds — lead that resists darts, camo that most towers can't see, and regrow that heals if you don't finish it.
Camo balloons are invisible to most towers unless something grants detection; lead balloons resist sharp projectiles and need explosives or fire. Both appear at set rounds, so you have to prepare before they arrive.
On corners and long straights, where a tower's range covers the track for the longest time. A tower beside a short straight section gets a fraction of the shots one on a hairpin does.
Yes — it runs in the browser with nothing to install.