Thanks for your reply, fellow Elf, but I had already figured out your two points :
1. I intuitively understood that a swamp would slow down the attackers, but I was seeking a more precisely quantitative answer :
What exact parameters does the game program compute when sieges occur in a swamp region, parameters granting the Elven defender which "huge advantages" ?
2. As I said in my original post, I did read the part of the region's description where bonuses (and penalties) are enumerated, but I did not notice there any bonus specifically associated with a swamp ecology.
Perhaps only a developer could answer my query, since game developers do have access to the parameters that are considered in the game program's code.