Dota 1 is still available and played by gamers and eSports fans all around the world. The game is a custom map for Warcraft III which means you need to have a copy of Warcraft 3 to play Dota1. You can download all the versions of the custom map at PlayDota.
The site also contains patch notes which the describes the balance changes made in a specific versions which might be good if you are looking to improve your game. IceFrog is always aiming for a well balanced game but since the MoBA game contains so many different heroes it is very hard to keep it perfectly balanced all the time.
But as usual with eSports games, the meta game develops, the professionals finds new strategies and counter-strategies, which puts the game in constant motion which makes the game so fun and perfect!