By default, the entities defined in the entity_default.txt (located in the \raws\objects\ folder) have
[PROGRESS_TRIGGER_POP_SIEGE:3]
[PROGRESS_TRIGGER_PROD_SIEGE:0]
[PROGRESS_TRIGGER_TRADE_SIEGE:0]
For the values, the pop_siege trigger :
A value of 0 disables the trigger. 1 corresponds to 20 dwarves, 2 to 50 dwarves, 3 to 80, 4 to 110, and 5 to 140.
the prod siege trigger
A value of 0 disables the trigger. 1 corresponds to 5000☼ created wealth, 2 to 25000☼, 3 to 100000☼, 4 to 200000☼, and 5 to 300000☼.
the trade siege trigger
A value of 0 disables the trigger. 1 corresponds to 500☼ exported wealth, 2 to 2500☼, 3 to 10000☼, 4 to 20000☼, and 5 to 30000☼.
So for all entities (and especially the ) changing to
[PROGRESS_TRIGGER_POP_SIEGE:1]
[PROGRESS_TRIGGER_PROD_SIEGE:1]
[PROGRESS_TRIGGER_TRADE_SIEGE:1]
May allow more sieges and earlier.
Though it will not make later siege more difficult as you'll still need to make intentional mistake in your fort defense (meaning you must not make an efficient fort, you'll need to leave some opening or not make too many trap or avoid making raising bridge to block or flatten invaders etc.. by example)
As said the ambushes are oddly gone, maybe re-adding the [AMBUSHER] tag to the [ENTITY:EVIL] may give the gobs their ability to ambush your dwarves again, in current version only [ENTITY:FOREST] (the elves) [ENTITY:SKULKING] (kobolds) and [SUBTERRANEAN_ANIMAL_PEOPLES] (the cave animal men) have it
In case you find invaders or specific big animals too passive, give them a [CRAZED] tag , it will make them berserk and they will attack on sight anything that isn't of their own specie.
This isn't for the entity_default.txt , it's for individual species (by example making gobs [CRAZED] will have the tag added to in the creature_standard.txt ).
Note about [CRAZED] , if you give the tag to a specie you want to play in Fortress mode (the dwarves by example if you didn't mod your game) , their fortress will crumble immediately and make you lose the game before even starting.
The problem does not happen in worldgen, [CRAZED] people will still build whatever they are set to build by the game, it's only a problem in Fortress mode.
Adding [CRAZED] to a specie that comes to your fort regularly as visitor may be interesting
If still not challenging fun enough, playing 34.11 is still a good option as there are some things that work differently and present an increased challenge (or 0.28.181.40d if you want to deal with the infamous river carps as they got unfortunately hugely nerfed after that version) (or go back to 23.130.23a, if you want to know why elephants were feared in the past and play a very different Fortress mode from what it is today as there were no z-levels)