The thing is, before this goes in, we need a way to reduce buildings to ruins through play, "onscreen". Currently, Megabeasts can reduce buildings to ruins in world gen, but that's all.
If you want to see a world where every structure is in ruins, incidentally, put this 'Monster Island' in your world_gen.txt:
[WORLD_GEN]
[TITLE:MONSTER ISLAND]
[DIM:17:17]
[EMBARK_POINTS:10000]
[END_YEAR:1050]
[BEAST_END_YEAR:30:80]
[REVEAL_ALL_HISTORY:1]
[CULL_HISTORICAL_FIGURES:0]
[ELEVATION:1:400:401:401]
[RAINFALL:0:100:26:26]
[TEMPERATURE:25:75:26:26]
[DRAINAGE:0:100:26:26]
[VOLCANISM:0:100:26:26]
[SAVAGERY:0:100:26:26]
[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:1:1:1:1:1:1]
[VOLCANISM_FREQUENCY:1:1:1:1:1:1]
[TITAN_NUMBER:1000]
[TITAN_ATTACK_TRIGGER:80:0:100000]
[DEMON_NUMBER:20]
[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:10]
[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:500]
[CAVE_MIN_SIZE:500]
[CAVE_MAX_SIZE:500]
[MOUNTAIN_CAVE_MIN:5]
[NON_MOUNTAIN_CAVE_MIN:20]
[ALL_CAVES_VISIBLE:1]
[SHOW_EMBARK_TUNNEL:2]
[TOTAL_CIV_NUMBER:4]
[TOTAL_CIV_POPULATION:100000]
[PLAYABLE_CIVILIZATION_REQUIRED:1]
[ELEVATION_RANGES:36:72:36]
[RAIN_RANGES:0:0:0]
[DRAINAGE_RANGES:0:0:0]
[SAVAGERY_RANGES:0:0:0]
[VOLCANISM_RANGES:0:0:0]
...this will create a pocket universe overrun with megabeasts, who will quickly kill all the races and reduce their cities to ruins.