Here are some ideas for Stardock on the types of buildings which can be placed/built inside a town.  Anything listed can have a balanced cost associated such as gold, magic, resources and/or time:

A} The Jail
The jail is where the player can capture champions or unique animals allowing them to be traded for gold, items or diplomatic treaties.  The animals can be domesticated for armies and some even used as mounted units.   For example let's say player_A is very powerful and my exploring army was lucky enough to find his most powerful champion alone and wounded from a recent cave exploration.  The player agrees to have his champion captured instead of being killed.  Once in my jail I request gold and a 20_Turn peace treaty for his safe return... or perhaps auction him off amongst the other players.

B} Marketplace
The Marketplace improves the percentage of gold income produced from the town where it's located.  Naturally this building is better for large towns which produce more gold and not a village on a desert island. An upgrade via gold & resources for this structure would be expanding a bank building next to the market which improves the percentage of gold produced. 

C} Engineers Guild
This provides the option to train some of the population to become engineers who can then build catapults, ballistas, battering rams, and other mechanical units.  At a cost the engineers can also build defensive traps such as burning oil, spikes, and caged wild dogs.

D} Storage Facility
This allows increased storage of resources within the town.  This probably already exists since a natural demand for this type of building would exist.  This building can be upgraded to provide more space being a 2 story building or even 3 story building via the cost of gold & resources.

E} Statue of a Nations Hero
Defending nation receives +2 morale for all units… only of value to the original builder.  Conquered towns would need to rebuild their own hero.

F} Blacksmith
Allows the forging of weapons for regular units and it can be upgraded by several different combinations.  Completing a quest is one method, enchanted by a spell is a second method, structural upgrades via gold & resources is a third method.  This would allow for units to evolve from carrying Clubs to eventually reach builds of carrying a Death Knight Axes of Strength or Holy Defender Bows of Resistance… naturally these last two examples should be extremely costly and time consuming to achieve.  The forging of items for champions should overall be allowed to become more powerful.

G} Channellers Magic Circle
Allows the Channeller to teleport into this location… only of value to the original builder as conquered towns would need to rebuild their own magic circle unique to their channeller.  This location can have structural upgrades via gold & resources so casted magic spells provide stronger and/or cheaper results.

H} Alchemists Workshop & Garden
Allows the growing of a few specific herbs which are random and depend on the type of terrain.  The herbs can be mixed together and/or with resources for developing potions.  Potions may provide a champion or channeller a temporary increase in one statistic or a cure for a poison or disease or a resistance to an element.  Each potion can only be used once.

I} Mages Library
Allows for a minor research bonus and a mage inside may spend one or more turns writing a magic scroll.  The magic scroll would be a spell the mage within the library could cast, but once on a scroll it can be casted once by any mage.  These scrolls have a magic cost and time cost associated with them.

J} Thieves Guild
Allows the option to train some of the population to become thieves, spys and/or assassins.  These units use stealth as their primary defense and it also provides advantages for surprise attacks.  Structural upgrades via gold & resources… first level provide thieves, second level provides spys, third level provides assassins.   

K} Stables/Ranch
Allows for the training of animals which can be used for combat.  A few larger animals such as horses can be used as mounts. Structural upgrades could allow for more wild animals to be tamed such as wolves and eventually bears.

L} Farm & Fields
Allows for the growth of food feeding the population and breeding of animals.  Structural upgrades could allow for additional fields generating more food for the town.

M} Shipyard & Docks
Allows for the building of sea vessels(Explorer, Goods Trader, Military type, Civilian Transport, Fishing Boat) .  Sea vessels can also be repaired and/or improved at this location.  Structural upgrades could allow for additional defenses against sea_based attacks as well as more evolved sea vessels.

N} The Keep
This is a major government/military building providing strong defensive and is usually located within the center of a town.  Structural upgrades can evolve from Keep>Fortress>Castle>Capital... naturally only one location can provide a Capital for each race.  Benefits from Structural upgrades can include stronger physical and magical defense for during battles at this location or spells which target this location on the main map.  Perhaps the channeller can have his own personal guards trained here as well.