Bloons TD (which is really BTD3) is great. Now first off, you obviously can't install this on most modern devices, you will need an old iOS device below iOS 11. So find an old iOS device for sale somewhere if you don't have one, since you cannot emulate this whatsoever.
Bloons TD 3 is overall the best of the three classic Bloons TD games before BTD4, building upon the fundamentals of the previous two. This iOS port is overall a faithful recreation of BTD3 on Web/flash, but with some new additions. Most namely, you get new map packs not found in the original flash game, although as a trade off two of the maps in the original are not included. These map packs are an intetesting idea I wish were used more in later BTDs, where themed maps would have similar themes, but could be used differently in different maps. Gameplay is what you would expect, build towers, pop the bloons, and you can even do freeplay and see how long your device lasts.
Why do I not consider this a 5 star experience? Simply put, unlike later BTDs, you have no sandbox mode, or any gamemode outside of the classic gamemode. So it can get old, but not before you finish all the maps. That being said if you have an old iOS device, this is worth the purchase.