When you wish to start a new game as an Elf, you are offered three ecological environments : mountain, swamp, forest.
The general description of the swamp ecology (located in the Erthee L'Bala region) declares :
Swamp terrain gives huge defensive advantage on home territory
Being a rather defensive builder (the contrary of an offensive warrior), this appeals to me, but I have no idea what "huge defensive advantage" exactly means.
Following the general description, the bonuses and penalties are precisely detailed, but I cannot figure out how that information represents a "huge defensive advantage", considering that there are penalties and that the word "huge" should correspond to something that can be precisely identified.