[HoneyBadger]

I wouldn't be in favor of adding buildings, except as Kristoffer states-the ability to strengthen units already present, by way of more advanced armament, equipment, special weapons and tactics, etc.

That would help in balancing, in one way, because you could have forces/troops in a given nation start weak or off-kilter (jotuns armed with daggers, for example) and then have a new set of improved troops replace the old ones upon the construction of whatever.

The building could have positive effect on resources and gold-production, administration, etc. but at the same time, be a clear and easily destroyed target, where as things stand, all that infrastructure that's assumed to exist, is off limits, save in very abstract ways (pillaging, etc.)

Perhaps, to simulate open trade, such a building, could only exist in areas without castles, replacing them and becoming even more vulnerable targets, or they could simply just improve gold/resources in provinces that lack a castle, losing this ability in castled provinces.

The first appeals to me in that you could have nations that don't have a fortress to start out, but do have access to the other building, making them more vulnerable to an initial raid, but giving them a boost to production in the early game.

The second appeals in that you could still utilize the first one, giving one or the other building at the beginning of the game, and granting the ability to build the other later on, and if such a building co-existed with a castle, perhaps it could then allow the construction of fieldworks and seige engines/defenses, and other abilities could become viable when a lab/temple is present.