When I was generating a world for my bridge-gatehouse-fortress look what I found.
Worldgen
Created in DF v0.34.10.
[WORLD_GEN]
[TITLE:CREATE WORLD NOW = 2555545]
[SEED:aeeGGiq0MsYWG4QMUAqg]
[HISTORY_SEED:GYCwqU6iMmEygIU0oOSi]
[NAME_SEED:8imOmyMuW4QO0IQWQuwI]
[CREATURE_SEED:WIIAOmsmMcwIsagAQuMu]
[DIM:33:33]
[EMBARK_POINTS:1324]
[END_YEAR:1050]
[BEAST_END_YEAR:1050:-1]
[REVEAL_ALL_HISTORY:1]
[CULL_HISTORICAL_FIGURES:0]
[ELEVATION:1:400:401:401]
[RAINFALL:0:100:51:51]
[TEMPERATURE:25:75:51:51]
[DRAINAGE:0:100:51:51]
[VOLCANISM:0:100:51:51]
[SAVAGERY:0:100:51:51]
[ELEVATION_FREQUENCY:1:1:1:1:1:1]
[RAIN_FREQUENCY:1:1:1:1:1:1]
[DRAINAGE_FREQUENCY:1:1:1:1:1:1]
[TEMPERATURE_FREQUENCY:1:1:1:1:1:1]
[SAVAGERY_FREQUENCY:3:1:1:2:2:3]
[VOLCANISM_FREQUENCY:1:1:1:1:1:1]
[MINERAL_SCARCITY:100]
[MEGABEAST_CAP:2]
[SEMIMEGABEAST_CAP:4]
[TITAN_NUMBER:2]
[TITAN_ATTACK_TRIGGER:80:0:100000]
[DEMON_NUMBER:20]
[NIGHT_TROLL_NUMBER:10]
[BOGEYMAN_NUMBER:10]
[VAMPIRE_NUMBER:10]
[WEREBEAST_NUMBER:10]
[SECRET_NUMBER:20]
[REGIONAL_INTERACTION_NUMBER:20]
[DISTURBANCE_INTERACTION_NUMBER:20]
[EVIL_CLOUD_NUMBER:10]
[EVIL_RAIN_NUMBER:10]
[GOOD_SQ_COUNTS:1:0:0]
[EVIL_SQ_COUNTS:1:0:0]
[PEAK_NUMBER_MIN:1]
[PARTIAL_OCEAN_EDGE_MIN:0]
[COMPLETE_OCEAN_EDGE_MIN:4]
[VOLCANO_MIN:1]
[REGION_COUNTS:SWAMP:0:0:0]
[REGION_COUNTS:DESERT:0:0:0]
[REGION_COUNTS:FOREST:0:0:0]
[REGION_COUNTS:MOUNTAINS:0:0:0]
[REGION_COUNTS:OCEAN:0:0:0]
[REGION_COUNTS:GLACIER:0:0:0]
[REGION_COUNTS:TUNDRA:0:0:0]
[REGION_COUNTS:GRASSLAND:0:0:0]
[REGION_COUNTS:HILLS:0:0:0]
[EROSION_CYCLE_COUNT:250]
[RIVER_MINS:1:1]
[PERIODICALLY_ERODE_EXTREMES:1]
[OROGRAPHIC_PRECIPITATION:1]
[SUBREGION_MAX:2750]
[CAVERN_LAYER_COUNT:3]
[CAVERN_LAYER_OPENNESS_MIN:0]
[CAVERN_LAYER_OPENNESS_MAX:100]
[CAVERN_LAYER_PASSAGE_DENSITY_MIN:0]
[CAVERN_LAYER_PASSAGE_DENSITY_MAX:100]
[CAVERN_LAYER_WATER_MIN:0]
[CAVERN_LAYER_WATER_MAX:100]
[HAVE_BOTTOM_LAYER_1:1]
[HAVE_BOTTOM_LAYER_2:1]
[LEVELS_ABOVE_GROUND:15]
[LEVELS_ABOVE_LAYER_1:5]
[LEVELS_ABOVE_LAYER_2:1]
[LEVELS_ABOVE_LAYER_3:1]
[LEVELS_ABOVE_LAYER_4:1]
[LEVELS_ABOVE_LAYER_5:2]
[LEVELS_AT_BOTTOM:1]
[CAVE_MIN_SIZE:5]
[CAVE_MAX_SIZE:25]
[MOUNTAIN_CAVE_MIN:2]
[NON_MOUNTAIN_CAVE_MIN:2]
[ALL_CAVES_VISIBLE:0]
[SHOW_EMBARK_TUNNEL:2]
[TOTAL_CIV_NUMBER:12]
[TOTAL_CIV_POPULATION:15000]
[SITE_CAP:136]
[PLAYABLE_CIVILIZATION_REQUIRED:1]
[ELEVATION_RANGES:36:72:36]
[RAIN_RANGES:0:0:0]
[DRAINAGE_RANGES:0:0:0]
[SAVAGERY_RANGES:34:136:272]
[VOLCANISM_RANGES:0:0:0]
Embark - it's place marked as bridge over brook (Where a road crosses The Mess of Fear brook
)
It's flat, warm, woodland, moderate vegetation, wilderness, brook, clay, some soil, shallow metals, deep metals, no aquifier. (sadly few murky pools)
Neighbours: Dwarves, goblins(---), elves, humans. No tower on whole island.
But now the interesting part. On 3x3 embark (141 z-levels) it has 9129 adamantine from 10 to 87 level. And it is safe to mine from level 68. Visible from one of the caverns.
As usual it has tons of silver but it also has 37000 units of native gold (for golden fort?)
On the other hand map goes like 2 levels of clay/soil then 2 levels of stone and then first cavern. Only good for aboveground fortress.
Also there is "only" cca 2000 of iron ores located only on 3 levels. (Maybe that is not that bad if you count in 100000 of marble)
Magma located only on levels 19-23.
And note that I stopped generating that world after 545 years if it has any effect.