Due to the progress made in this thread https://forums.elementalgame.com/394884 I wanted to post something I tinkered with as a result.

What we have here is essentially a summonable defensive structure that doesn't need a city. For this example I made it a spell, but other variations are possible (such as having an item used by a champion produce a building).

In particular, two gamemodifiers are of interest

        <GameModifier InternalName="PlaceTerrain">
            <ModType>Map</ModType>
            <Attribute>PlaceTerrain</Attribute>
            <TerrainType>HF_PermanentIce</TerrainType>
            <Value>0</Value>

        </GameModifier>
        <GameModifier InternalName="SummonFlowers">
            <ModType>Map</ModType>
            <Attribute>CreateWorldProp</Attribute>
            <StrVal>HF_Icefortress</StrVal>
        </GameModifier>

The first modifier creates the custom-made terrain, which actually gives the tile its icy effect on the ground, and the defensive bonuses. Note that other bonuses are entirely feasible, such as increased sight for watchtowers, increased health regeneration for inns, increased attack for an ambush tile. Note that the modifiers for defending the tile is actually in the terraintype, NOT the spell.

The second modifier is just the tile. It can look like anything you can make in the tile editor.

So, what are the flaws of this method?

- If you cast it on a resource that is unbuilt, the resource is destroyed

- We can't give it an upkeep cost

- We can't make it disappear after X amount of turns (we can make the tile design disappear, but the terrain bonuses will last)

- If the modifiers are linked to an item, and you use it in your own town, you will destroy your own town

- AI probably won't attack you, since it DOES alter your combat rating

So, what can we do?

- Add custom terrain modifiers to better represent defense bonuses

- Limit the spell to forest or hill tiles, or make it only cast on land

- Define what tactical map is to be used when fighting in the tile (also possible to link to your own tactical map)

- Replace the tile with another terrain spell, or a city building

- Make it unpassable for ground units (unknown effects on occupying unit)

- Make it slow to pass

- Make it damage troops on the tile (AI probably won't notice) or provide negative bonuses instead of positive bonuses

- I think we can make it summonable only inside our influence, but I'm not 100% sure

- Make it indestructible to town building

These are all the files necessary, if you want to check out the XML.

http://www.mediafire.com/?im0620gv99cn78n

For the end user, it's probably not a good idea to use this mod if you're using any other mods.