A few of the rituals I know you will need. This is going off the basis that you would want to use magic for much of it, to greatly speed up the creation time.
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Create Floating Island, Cr(Mu)Te level 50: Create a 14 mile diameter island of pumice, with three integrated Sea Gates and more resistant to damage. This can generate a Preternatural Tether 1. (TME, p. 91)
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Create Back Fill, CrTe level 30: Creates up to 1 billion cubic paces of dirt (you need 477 million), which consist of 33 paces depth of clay with 2 paces depth of top soil. If desired some of the excess can be used to create hills. Clay is just better all around than dirt (water proof, allows easy cisterns). (TME, p 59)
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Creation of Verdant Grassland, CrHe level 40: This creates 120,000 acres of grass, more enough to cover the island (which is 98,520 acres). It includes +1 magnitude to include a mix rather than being just one type, which would allow you to create individual fields. Of note, wheat and grains are types of grass. Something like grain crops in one sector and grass/grazing land in the rest (this includes grasses common to forest in the forest section). Figuring 30k acres of cropland you would need between 250 and 3,000 peasant families to work this much land depending on the crops raised. This has the potential of some 1,000 pounds of income, before expenses, every year if fully staff (50 knight manors worth). This only uses up a little more than 90% of the land in one sector (30,000 acres). (TME, p 62)
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Creation of a Forest, CrHe level 60: Creates a forest of 1 million mature trees. It includes +1 magnitude so the exact mix of species can be selected, rather than being just one type. Cast after creating the scrub grasses that will provide the ground cover in the forest. This can generate a Preternatural Tether 2. (TME, p. 62)
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Rituals for creating the Roads & Towns
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4x Creation of the Well-Appointed Town, CrTe level 45: Used to create the towns around each great tower, the spell creates 800 well-appointed cottages (with plumbing for water, sewage, and hypocaustic systems) along with the roads between them (which contain the water, sewage, and hypocaustic pipes). The cottages are not identical, but have a range of layouts which makes the town seem less regimented (along with allowing the ability to make some stores or workshops). For an additional Magnitude you could create up to 8,000 cottages, though I believe that would be far more than you need. (TME, p. 61)
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A Network Reminiscent of Rome, CrTe level 35: This creates a road network of up to 378 miles total length. The roads are 3 paces wide, crown slightly, and are roughly 0.5 paces thick. It has an added complexity to include pipes for water, sewage, and hypocaustic systems to tie in with the next spell. At this level the spell will give you enough to have a double wide road around the outer circumference of the island, a road on each side of the waterways from the outer towers to the middle tower along with another ~206 miles for branches and bridges. Seems like a lot of excess but it would be quickly used up laying out roads through the farmland and grassland sections, along with adding a few bridges over the waterways (adding bridges does not require another magnitude) (TME, p. 60)
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Some variation of A System of Narrow Canals or Canals that Accommodate
Seagoing Vessels, CrTe level 25 or 30: This creates the stone lining for the three waterways and the lake around the center tower. No bottom is needed because of the clay fill. Would suggest adding +1 Complexity to include gates and fresh water pickups (to feed into the water system). Sewage should be dumped into the saltwater outer bays. (TME, p. 61~62)
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Rituals for creating the four "Great Towers"
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4x Conjuring the Great Tower, CrTe level 55: This forms the foundation and first two levels of each of the great towers. This can generate a Preternatural Tether 1 each casting. (HP, p. 39)
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8x Heights of the Great Tower, CrTe level 50: Two of these per tower will create floors 3~10. This can generate a Preternatural Tether 1 each casting. (HP, p. 40)
Each of the towers will be 2,000 feet in diameter and 200 feet high after this. Rather than trying to make them any higher than this, I would instead recommend using a modified version of Conjuring the Mystic Tower to create smaller "sub towers" atop them which can be used by individual Magi as their sanctums. You could also install gardens and pools atop the towers.
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Some additional Ritual Effects to consider
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A version of Conjuring the Mystic Tower to create utility spaces (with a higher percentage underground) for the areas not near the Great Towers. Can be used for things like winery with wine cellars, slaughter house with larders, magical furnaces, hypocaustic systems, and catacombs. The hypocaustic system would be nice if you were traveling in a colder area.
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A "tower" emerging from the bottom of the island. This would serve as a keel and could have "air locks" (opens with wards against salt water) allowing easy access under the island. This might prove to be a very useful means of gather Vis, since the island itself will mostly not produce any. (baring YSMV things like transplanting Trees/Plants of Virtue to it).
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To help the Aura form faster, consider Feeding the Font of the Covenant (or if you want to greatly increase the chance of a Regio than Stratifying the Subtle Realities). (RoP:M, p. 14)
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Other ideas
- In one of the tower towns, install a Hermetic Shipyard. This would allow you to build, repair,and maintain ships which could help support the island. Instead of just focusing on enchanting ships, consider creating enchanted items that improve the quality and capability of the actual shipyard. If you want some ideas, I have posted several times extensively on the range of items used in my own game which help with this. Here is one.