I would never turn specific terrains into barb zones like that, particularly when we have several civs designed towards those terrains. I prefer a more emergent setup; Barbs became strong in those regions due to lack of settlement. The same applies to regions blocked off via mountains, or on different continents, etc.
As for the siege... We actually have two siege unitcombats planned. One will be along the lines of what you are saying, though they will NOT have a high attack strength.
Does sound like you had a fun game though.
Fair enough.
On another note, what about "Big Bads" with a Variable leash around a lair?
to tie into the Lair upgrade (over time) system ... Lair 1 pops out of thin air (usually at game start) and has a Medium Bad with a very tight leash.
Lair 1 turns into Lair 2 over time.
Big Bad pops up when lair becomes "Lair 2". Big Bad has a longer leash and more variable leash. Medium Bad doesn't go away, its just that Big Bad is more dangerous and can travel farther from nest.
Lair 2 (sometimes) might turn into a Lair 3 (over a considerable amount of time).
Once Lair 3 is created from L2, a "Super Bad" pops up, that practically sits atop its lair. (with maybe a 1 tile leash)
Therefore, the short leash on tier 1 prevents a leash from being too long and possibly intersect a Settler Starting Position. However, the Tier 2 will provide increased danger and incentive to kill the lair. Then, Tier 3 will represent a fully matured lair (relatively) that is near impossible (or very hard) to destroy. Due to its proximity to the Superbad. Maybe even a Tier 3 lair could need a separate spell to destroy it, allowing you to need 2 turns to kill a tier 3 lair (all but necessitating either killing the super bad or bringing at least 3 units to the location. [at bare minimum])
Also, perhaps minion lair spawns (like Skeleton Warriors, Lions, Wolves, etc) could have a 5% chance [some low percentage] to be leashed to the lair, yet at a good radius, say 10- 15 tiles or so. While the rest roam freely. (assuming all minions have the ability to enter cultural borders of civilized nations).
Another interesting thought, although I'm not sure how easy or difficult it would be, would be for Big Bads, or at least Super Bads, to have a low-medium % chance to gain the "crazed" promotion if a Civilized unit where to attack it. ergo, Civ A's warrior attacks the dragon. Dragon gains crazed. 10 turns later Dragon gains enraged promotion and exits its leashed zone to attack the city. After killing a spearman, Dragon returns to its leash zone ... but now its crazed so it will start doing this regularly ... further requiring the civ to now really exert itself to kill the Dragon nest, or to simply ignore it and feed it fodder troops.
But either way, an attack upon the Dragon's nest (or, well, the Dragon specifically) caused it to be angered, and semi-regularly go out for a destruction fest.
Alternatively, instead of crazed it could be a new promotion which has a small % chance of wearing off and applies the enraged promotion (or something to exit the leash and be aggressive) more frequently (yet still random.)
If you're still with us, I'd like to hear your ideas about my ideas. Thank you.