I loved Civilization V, but it was not a game without its problems. The longer you played it, the more it became apparent that the AI wasn't quite as sharp as it should have been. This enormous patch should help.
It's simply too vast to drop in its entirety here, so simply know that almost every single part of the game's artificial intelligence has been modified, from how it deals with other nations to how it explores to - most importantly - how it fights you.
Joining the AI tweaks are a number of additions to the game, including new national wonders and, my favourite of the lot, a "New "Angry Genghis" loading screen (replaces the "fluffy-bunny Genghis" loading screen)".
Only some serious time with the game will tell if these make massive improvements to it, so off you go, get testing! You can read the complete list of changes, tweaks and additions at the link below.
Civilization V [Steam]