Hey,
I think it would be nice to be able to assault a Goblin fortress, If you have enough soldiers.
I think the best way of doing this is in the same style as master of Olympus Zeus, where you select soldiers to go and attack the fortress, and they go off and do that while you stay in control of your fortress, several months later (depending on how far away the fortress is and whether or no the attack was a success) your troops come back, except in a few, very sad cases.
too add another level to this, rather than just having to send loads of troops, it could also be that you select a commander, to lead the attack. This leads onto another skill - command. To get more experience in command soldiers simply have to be the leader of their squad, and over time it accumulates, as well as bonuses if their squad members get kills. The higher the command skill the more likely it is that the attack is a success.
Once soldiers come back from an attack, you get a report of the casualties and how the attack went, etc. There could even be three ways the attack could go:
Victory! The halls of the Goblin fortress are filled with the vile corpses! You now have the option to send a group of migrants to hold the fortress, and send back a quarter of whatever they make.
Stalemate! Both sides have taken heavy losses, but the fortress is still there for the taking. Please send more dwarfs.
Defeat! The attacking force has been repulsed, and limped back home... People depressed by "Recent military defeat".
Sound good?