Some time ago, DVNO proposed the following as a solution to the sparseness of sieges, as seen in the quote below:
For some reason in the entity files, PLAYER_FORTRESS is a functional site tag but no one uses them, not even Dwarves. Can setting the Goblins to only have a preference for player fortresses aid with the Siege drought in 40.24?
setting [LIKES_SITE:PLAYER_FORTRESS] and removing all other preferences, even tolerance for an Entity's own default site, got me *a* siege in 40.24 at least. I don't know if that was just astronomical odds that I happened to get my only siege in six RL months with 40.24 while using those modded raws or not. Could never get it to happen again, but I haven't been playing DF much since then.
Some independent testing would be nice to have, I'd like to know if I'm going crazy or not.
As far as I am aware, sieges in the current version seem to be influenced by distance and whether or not the entity in question has a particular preference for the target site. If I said anything that is known to be wrong, please do say so.
If what I said above is correct, it may explain why there are so few sieges. Player sites use the PLAYER_FORTRESS token, rather than that used by the rest of the entity they are playing as. So, as suggested by DVNO, making an entity like PLAYER_FORTRESS will increase attacks. Or so I hope. I've yet to see any real testing on this, so I decided to try it myself.
What I am going to do is take the fortress defense mod, as it currently stand in its thread in Mod Releases, and add [LIKES_SITE:PLAYER_FORTRESS] and [TOLERATES_SITE:PLAYER_FORTRESS] to each entity. I will also do the same to standard DF humans, elves and goblins. I will then create two fortresses near as many civilizations as I can, which I will then try to keep alive for at least a few years. After those two are done, I will create another two, but this time in a position that is as far away from civilizations as possible while having them still remain in the neighbors menu. I will later compare it to a non-modded DF with only the changes stated above, and afterwards to a completely unchanged DF.
I will post the results here as they appear.
EDIT: The world was generated with advanced worldgen, medium region. The only changes were a 100 years of duration, minimum cavern water to 5, 2000 embark points and mineral scarcity to 1000.
As a bonus, the civilization I picked is dying to the point I got a warning about it.