I began to think about how to make the walls modular. I exported one of the wall modules to 3DS Max and then split it in two the height, as I thought the rocks had to be big enough for a temple. Then I made a pattern consisting of 8 different rocks in size. Once texturized, They were brought into UE4 as single meshes, to edit them within different prefabs to alter the patterns of each wall module made of 2 levels of rocks.
Then I just had to duplicate and place the other walls, following the old wall mesh pattern, and also duplicate in height direction. In order to deal with some gaps I placed Prefabs on the corners too, which will be editted after, removing unnecessary blocks.
Finally, made the Main Door and and the side corridors until I had the complete result except for the ceilings.
Next steps are the ceilings, ground and lighting.