A casual tower defense game that can be hardcore if you like.
With a wide variety of towers and heroes great in excelling in their specialties, maps ranging from casual to challenging they all can be beaten with strategy skill (and spam cus why not spam a dozen towers of the same type cus the more there are, the greater the damage), and with events and daily challenges ranging with a different variety of skill, BTD6 has all that you need.
Even the community pulls through with their own versions of the game like custom map creators and custom challenges filled with modifiers and many more.....
If you are starting to play the game and don't know what to do or how to use the towers more efficiently there is a lot of content in YouTube that will help you familiarise youself to the world of BTD6. However I do recommend only checking out guides for for gaining tower XP and beating Boss events(Yes there is Boss events in this game and you better be prepared for them if you are to try) an play the main core maps completely blind which will help see your mistakes for failing a round and how to be better at it.
For multiplayer there is a lot more. Trophy stores for customisation and to keep games fresh and though no live ones there are multiple events for it like Races(Finish the map as quickly as you can by letting some rounds start early but not too much!), Contested Territory(Claim and defend different tiles against others creating a high score) Boss Events(Bloons so deadly that even the BAD respect them out of fear), Odysseys (Beat different maps in order for great treasures...) and Collection Events where you can collect some tokens either eggs or snowballs for instant monkeys, a power up that can help you) and best of all these are hard earned no in app purchases for these.
There is more but I'll think it's better for you to find out. The best part is that the game receives constant updates an few months to introduce map towers heroes (and Paragons.... I'll let you find that one out).
Though the game requires an online connection to sync your progress to the cloud and access events you can play it offline aswell. Overall game is an excellent tower defense game so if you are a TD fan, BTD6 will not disappoint.