open source ?
errrr. UDK ? i don’t know it but i think ( wich might not be true ) it’s open…
Why do you want it open source ?
ye, this is a stoopid question as i guess you got reason for wanting this ^^
Personnally i use unity.
For this: a FPS walk in historical places: Carcassonne Medieval city
BGE is also a possibility… Even if i’d say its worse than U3D in a game-creator’s point of view.
game engine…
U3D
UDK
Cry
Monkey
…
I’d say EEVEE but never tried it and i think it’s more a renderer than a game engine…
It basically depends on what you wanna show ^^ and do 
happy blending !