I like the guild idea, its well thought out, detailed and different from other ideas presented.
I agree with the other posters that city building as it is needs to be improved. Now I can afford to build everything for every city and every city looks alike. In the LOTR and SW movies each city had a unique look.
Perhaps a city could build improvements linked to Earth OR Fire OR Water or AIR
A sovereign with access to Earth Magic could have an option to build
Great Tree (build one of two options) where the player can build a Tree of Life (Maint +2) or a Tree of Death (Maint +2)
- Tree of Life (think Avatar) that would tower over the other buildings and open up research options for more Earth Magic / Nature Spells along with giving defending units in the city healing of +3hp per tactical combat round/season OR
- Tree of Death (Vines) that covers the tactical battleground in vines that reduce the movement of attackers by 1 unless they resist each turn and opens up more research branches for Earth Magic/Nature Spells. This lengthens the time it takes to close with the defenders and exposes them to more archer attacks.
Scorpian Tree Fortifications - scorpians on large trees that shoots arrows at attacking units (i.e. bonus non-moving defensive archer unit). You can build this several times, each one built gives the city an additional defensive non-moving scorpian unit. (Maint +1 per scorpian)
The scorpio was a smaller catapult-type weapon, more of a sniper weapon than a siege engine, being operated by only one man. The scorpio was basically a primitive giant crossbow, a "catapult with bolts", probably first invented by the Greeks, then later adopted and used on a larger scale by the Roman legions. This catapult used a system of torsion springs, making it possible to obtain very great power and thus a high speed of ejection for the bolts.
Forest Wall - that triggers a special tactical map for defenders of this city (Maint +1). Forest wall funnels the attackers into a narrow access point and exposes them to attack from several defending units. (Maint +1) Forest wall sections can be destroyed by fire or damage.
Ranger School - Allows a trait that allows units to enter tactical combat forest tiles (not allowed to normal units) where they receive a +10 bonus to dodge. (Maint +1)
Earth Academy - Opens research branches for new earth spells. (Maint +1). If razed you lose access to these research branches and any uncompleted research investment in these branches.
A sovereign with access to Water Magic might have an option to build:
Dam (if adjacent to a river) - The dam can be opened in combat to flood the battlefield giving access to a new tactical map with impassible water covered sections that funnel attackers into a choke point. (It takes 4 seasons to recharge a dam for use again) (Maint +2, and it generates +3GP & -10% unrest per turn if charged (i.e. not opened and not recharging).
Moat - On the tactical map attackers must cross a moat to reach the defenders. The moat reduces their movement by 1, giving the defenders an extra turn or two to organize their defenses. Units in Plate cannot cross the moat. (Maint +1)
Water Academy - Opens research branches for new water spells (Maint +1)
Elemental Pool - Summons a Guardian Water Elemental if the city is attacked
Canals - A city next to a water source; river or ocean city is surrounded by canals. This changes the tactical map to show canals (water barriers) and bridges that limit the access routes of any attacker. Attackers in Metal Armor (chain and plate) cannot move accross the water barriers but they can cross the bridges. (Main +2)