More secrets and longer worldgen history is what have the largest effect.
You should try to keep a reasonable balance between the different species, since not all races can become necromancers.
Some sub terrain species can become necromancers, although more rare, so having caves helps.
The towers themselves, seem to best form in regions whit medium savagery
Some people have luck whit maxing out number of civilizations, although that never worked well for me.
But worldgen setting can only take you half of the way. For fortress mode, there also seem to be quite a lot of luck involved. Some worlds can have a good amount of towers, but due to the low embark range, it can still be difficult to find areas where you are in range of several at the same time.
I had the best result whit these worldgen settings. Using small region to keep the worldgen time quick, while still having all the features. After the first 100 - 150 years, I pause and take a look at the map. If the towers that already formed are too spread out, I abort and try again.
Here is an example of a worldgen that went quite well, whit most of the towers ending up in one corner of the map. Embarking inside the square, you can be in range of about 10 towers + the standard 4 civilizations.

Worldgen used
[WORLD_GEN]
[TITLE:SMALL REGION SUPER TOWER]
[DIM:65:65]
[EMBARK_POINTS:1504]
[END_YEAR:500]
[BEAST_END_YEAR:240:80]
[REVEAL_ALL_HISTORY:1]
[CULL_HISTORICAL_FIGURES:1]
[ELEVATION:70:400:3200:3200]
[RAINFALL:0:100:1600:1600]
[TEMPERATURE:30:40:1600:1600]
[DRAINAGE:20:100:400:400]
[VOLCANISM:0:100:3200:3200]
[SAVAGERY:0:100:800:800]
[ELEVATION_FREQUENCY:4:1:0:0:0:1]
[RAIN_FREQUENCY:4:1:0:0:0:1]
[DRAINAGE_FREQUENCY:4:1:0:0:0:1]
[TEMPERATURE_FREQUENCY:4:1:0:0:0:1]
[SAVAGERY_FREQUENCY:4:1:0:1:0:1]
[VOLCANISM_FREQUENCY:4:1:0:0:0:1]
[POLE:NORTH]
[MINERAL_SCARCITY:500]
[MEGABEAST_CAP:9]
[SEMIMEGABEAST_CAP:12]
[TITAN_NUMBER:6]
[TITAN_ATTACK_TRIGGER:80:0:100000]
[DEMON_NUMBER:22]
[NIGHT_TROLL_NUMBER:22]
[BOGEYMAN_NUMBER:11]
[VAMPIRE_NUMBER:11]
[WEREBEAST_NUMBER:11]
[SECRET_NUMBER:1000]
[REGIONAL_INTERACTION_NUMBER:22]
[DISTURBANCE_INTERACTION_NUMBER:22]
[EVIL_CLOUD_NUMBER:11]
[EVIL_RAIN_NUMBER:11]
[GENERATE_DIVINE_MATERIALS:1]
[GOOD_SQ_COUNTS:250:0:0]
[EVIL_SQ_COUNTS:250:0:0]
[PEAK_NUMBER_MIN:50]
[PARTIAL_OCEAN_EDGE_MIN:0]
[COMPLETE_OCEAN_EDGE_MIN:0]
[VOLCANO_MIN:10]
[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:800]
[RIVER_MINS:0:0]
[PERIODICALLY_ERODE_EXTREMES:1]
[OROGRAPHIC_PRECIPITATION:1]
[SUBREGION_MAX:2500]
[CAVERN_LAYER_COUNT:3]
[CAVERN_LAYER_OPENNESS_MIN:80]
[CAVERN_LAYER_OPENNESS_MAX:100]
[CAVERN_LAYER_PASSAGE_DENSITY_MIN:10]
[CAVERN_LAYER_PASSAGE_DENSITY_MAX:20]
[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_1:12]
[LEVELS_ABOVE_LAYER_2:5]
[LEVELS_ABOVE_LAYER_3:5]
[LEVELS_ABOVE_LAYER_4:2]
[LEVELS_ABOVE_LAYER_5:2]
[LEVELS_AT_BOTTOM:1]
[CAVE_MIN_SIZE:2]
[CAVE_MAX_SIZE:4]
[MOUNTAIN_CAVE_MIN:50]
[NON_MOUNTAIN_CAVE_MIN:50]
[ALL_CAVES_VISIBLE:0]
[SHOW_EMBARK_TUNNEL:2]
[TOTAL_CIV_NUMBER:20]
[TOTAL_CIV_POPULATION:15000]
[SITE_CAP:264]
[PLAYABLE_CIVILIZATION_REQUIRED:1]
[ELEVATION_RANGES:0:0:0]
[RAIN_RANGES:1500:0:250]
[DRAINAGE_RANGES:250:0:1500]
[SAVAGERY_RANGES:0:0:0]
[VOLCANISM_RANGES:0:0:0]