"Avalon no Nazo, the sequel of Tears to Tiara takes place right after the defeat of the Erin and Albion kingdoms." It is an sequel! Thanks for the link Aeliana! I'll be definitely buying this one later ;D
It's a Gaiden ("Side Story") but I'm not sure if it takes place before or after the events of the first game, I've never personally played either of them. But if you want to buy Avalon no Nazo, it's pretty cheap:
Well, there are only two, Tears to Tiara: Kakan no Daichi and Tears to Tiara Gaiden: Avalon no Nazo. Avalon no Nazo has trophies, Kakan no Daichi does not. So I'd recommend Avalon no Nazo with the caveat that it is supposedly a fairly long (100+ hours) Plat.