This list is incomplete, you can help by expanding (or reducing!) it, the usual:
Important:
Steel Watchtower : IMPROVEMENT_STEEL_WATCHTOWER : Stone Watchtower (is very confusing!)
Medium:
~~Beacon : IMPROVEMENTBEACON : Beacon imp. art unused; merge into Reef (Beacon) feature art!
Lighthouse : IMPROVEMENTLIGHTHOUSE : Lighthouse imp. art unused; merge into Reef (Lighthouse) feature art!~~ (Fixed by Toffer)
Suburb : IMPROVEMENT_SUBURBS : Town
Meh:
Fossil Dig Site : IMPROVEMENT_FOSSIL_DIG_SITE : Forest Preserve
Archaeological Site : IMPROVEMENT_ARCHEOLOGICAL_SITE : Forest Preserve
Submerged Town : IMPROVEMENT_SUBMERGED_TOWN : Final Frontier spaceship; shares icon with unit
This list is incomplete, you can help by expanding (or reducing!) it, the usual:
Important: Steel Watchtower : IMPROVEMENT_STEEL_WATCHTOWER : Stone Watchtower (is very confusing!)
Medium: ~~Beacon : IMPROVEMENTBEACON : Beacon imp. art unused; merge into Reef (Beacon) feature art! Lighthouse : IMPROVEMENTLIGHTHOUSE : Lighthouse imp. art unused; merge into Reef (Lighthouse) feature art!~~ (Fixed by Toffer) Suburb : IMPROVEMENT_SUBURBS : Town
Meh: Fossil Dig Site : IMPROVEMENT_FOSSIL_DIG_SITE : Forest Preserve Archaeological Site : IMPROVEMENT_ARCHEOLOGICAL_SITE : Forest Preserve Submerged Town : IMPROVEMENT_SUBMERGED_TOWN : Final Frontier spaceship; shares icon with unit