Those heavy iron beauties in the picture above are examples of a Rodman Gun. They were designed during the Civil War by Union artilleryman Thomas Rodman. The ones above are located at Ft. McHenry in Baltimore.
The main innovation with these pieces was in the way they were cast. Traditionally artillery pieces were cast as one solid piece with the bore drilled out after cooling. This solid piece method meant that as the piece cooled, it did so from the outside in. This allowed small cracks and imperfections to form. While many of these imperfections would be taken care of during the drilling of the bore, there was always the possibility that others existed.
The Rodman method consisted of casting the piece as a hollow tube with a cooling tube in the center. This allowed the metal to cool from the inside out, which allowed for it to be stronger with fewer imperfections. Here is an article that gets into some of the small details. Basically, it made the gun stronger and allowed for heavier projectiles to be fired.
This casting method became the standard during and after the war and Rodman Guns were produced in many different sizes. Attempts were made to cast the unit as rifled pieces, with the spiral grooves in place, but it was not very successful. Later on, most of the guns were rifled.
The cannons above could fire a projectile weighing up to 444 pounds close to a mile. With that kind of power and distance, they became the go-to for coastal defense. Though several thousand of this style of artillery were made during the Civil War, very few if any actually were fired in combat. The two in the picture above in Baltimore harbor have only been fired for holidays and special occasions.