I could only go back after i finished the questparts in vergen. I run a straight hour in circels to find a way out but no chance. After finishing the questparts (get the sword of saskia i.e.) there was a purple marker who guided me when i pressed the left thumbstick and a shiny cloud appeared in the middle of the fightingfield.
I tried going back as well and couldn't. You have to complete your objectives in Vergen to go back.
Best you can do is sell some stuff to that elf girl. iirc she appears after you complete a Quest or two. --- Few Favorites- Ninja gaiden 1-2, Colony Wars 1-3,Xenogears,MarioKart,Lunar 2 GTA:SA,S&K,FF6-7,Zanac,S.Metroid,SMB2,NFL2K5,Halo 2, NG2,Vanquish,Witcher 2.