The Culverin is not very effective against anything other than artillery and ships, but makes up for this fact by being extraordinarily good at its two primary functions and can protect a large area. In comparison to other units, it deals more damage versus ships than the Iroquois Light Cannon (400 vs 210 unupgraded). While it doesn't match the extended range of the Mortar or Morutaru, it is far more accurate against a moving ship.
Unless an opponent is building cannon or a strong navy, Culverins have little use in most games. Some may want to keep a few around just in case of a ship or artillery attack, while others would consider it a waste of resources to build a unit which doesn't counter anything currently in play.
Due to their sizeable range, Culverins can adequately defend a shoreline from most ships as they have more range than all ships except Monitors and Ironclad. Not even the ships using Offshore Support or European Cannons will have a greater range than a Culverin, they come up one point short of matching its range. When combined with the even higher anti-ship range of Outposts and Town Centers built near the water and walls along the shore (preventing troops from being unloaded), the player will be safe inside their turtle shell until Monitors are able to be produced by the enemy.
However, with limited capability for aggression, its defensive powers does not tend to see much competitive use in 1v1. Due to its high 400 coin cost, in a short game a player able to produce culverins would generally rather spend it on more versatile methods for dealing with enemy Falconets anyway, such as cavalry. Units such as the Hussar can be repurposed on the offensive, while the Dragoon is more mobile. However, the culverin remains a useful defensive option in other game modes such as the campaigns and teamgames.
Fully upgraded Culverins being influenced by the SpanishUnction aura will very rapidly (1-2 shots, 456 damage versus artillery) destroy any artillery that lacks comparable or superior technological advantages. These same Culverins are also capable of inflicting 1710 damage to ships in a single shot, making it very difficult to successfully land an invasion force directly on Spanish territory, even when transporting troops in a high hit point vessel.
Asian civilizations can get Culverins by allying with Portuguese and Russians at the Consulate which provides armies and technologies that includes Culverins listed below. Unlike their regular counterpart, these Culverins are automatically upgraded in the Industrial Age and the Imperial Age (see here for the exact values).
Artillery except Mortars get the Case Shot attack (used against enemies within half of the normal range or closer, which deals 10% of the normal damage with 7 projectiles and +2 AOE, but with a multiplier of ×0.5 against cavalry and shock infantry, and also a ×2.0 multiplier against infantry for Culverins)
The Culverin gets its name from the Latin word for serpent - colubra - used to describe the cannon's distinctive curving handles. It was a long, heavy cannon and could fire at great range. A skilled team of gunners could use the Culverin's extreme range and precision to target specific enemy buildings or defenses.
A Culverin in Bombard mode
A Culverin in Limber mode
An Imperial Culverin Royale in Limber mode
A Culverin Royale in Bombard mode
A real-life culverin
A Culverin side view
Culverin in the original version and Definitive Edition