History and architecture? By architecture, I'm taking the liberty of guessing you mean 'of the historic kind'.

Athens, Paris, Rome, Istanbul, Firenze, Prague, Copenhagen, Stockholm, Brussels, Amsterdam, Budapest, Vienna ... and that's just a small selection from a single continent.

Need to narrow your preferences down a bit more.

Edit: Just noticed you said August. In that case, strike Paris, Rome, Firenze and Istanbul off the list. Too crowded, too hot, too many tourists. In fact, August is just about the 'worst' month for historical touristing in Europe.