The Monitor has a mortar fixed on the front for bombarding beaches or fortifications, but is weak against other ships and anti-ship units. It has an ability where it fires a more powerful shot that covers more range. The Monitor is most effective against buildings, as it deals 10 times as much damage against them compared to units. It best works against Forts, though any building that is deemed a threat can also be targeted. Monitors may also be acquired via the TEAM Monitor cards.
The Monitor is generally considered better than the Ironclad for long-range attacks. With the European Cannons or Offshore Support cards, Monitors can outrange all other ships. However, this range bonus only applies to its basic attack; the long-range attack is a fixed amount and damage versus ships is low (unless modified via the tactics discussed below).
If a puddle of water is large enough to build a dock, then it can usually hold a Monitor. This can grant easier control over a large area of a landlocked map, and obliterate or severely weaken any enemy units that encroach on the territory. Hot Air Balloons or the Spyglass ability can be used as a spotter for bombarding targets behind enemy walls that prevent passage of normal units.
Total tactic cost
The following includes Dock, Monitor (2), and upgrade costs. It does not take into account Spanish Unction, TEAM cards, or minor native civilization cost reduction cards for buildings and upgrades. It also does not include the Admiralty card for a third Monitor, which would add 800 wood, 200 coin to the costs (or subtract that, if a shot every 60 seconds is satisfactory). Keep in mind that the area of effect is very large, so when targeting tightly packed buildings click on the ground, not an individual building, to better spread the damage to multiple buildings. With armies, target a unit in the middle rather than the edge of the formation, or the area between the frontline and supporting cannons, for maximum effect. Unless leading a moving enemy army, in which case, use a best estimate.
2,550 wood, 1,150 coin for all Monitor damage upgrades in the Industrial Age (including Improved Warships card). 350 damage per long-range shot, one shot every 30 seconds if staggered.
4,050 wood, 2,650 coin with the Imperial Age upgrade. 450 damage per long-range shot, and one shot every 30 seconds if staggered.
Note:Water Dance for the Aztecs and Iroquois, and Fire Dance for the Sioux can not be used to buff a Monitor, as maps with such puddles of water do not support naval unit TEAM cards as they lack a water delivery point.
The highest attainable damage for the Monitor's regular attack is 339.04 (3,390.4 vs buildings, 67.808 vs villagers), and the long-range attack is 847.6 (8,476 vs buildings, 169.52 vs villagers) in 4v4 games. It requires all upgrades and damage cards, including Spanish Unction, and a British (Monitor Combat), Chinese (Improved Warships), and Indian (Shivaji's Tactics) ally sending their TEAM cards. British Monitors will inflict 228 regular damage, and 570 long-range shot damage. India 224 regular damage, and 560 long-range. China 216 regular damage, and 540 long-range damage. With timing practice, such a team can lay down rapid bombardments in a large number of areas simultaneously via staggering the long-range shots of their ships while the Spanish focus on the enemy armies exclusively.
The four civilizations also synergize well in another significant way explained here.
Long-range Bombardment Attack: Fires a single mortar shell to the target from a range of 100, doing 200 (2,000 against buildings, 40 against villagers, 20 against Docks) siege damage in an AOE of 10. 60 seconds cooldown.
"Monitor" actually refers to small turreted warships of post 1860s, but carrying disproportionate large guns. The correct name for this unit is thus "Bomb Vessel" or "Bomb Ship", although the Monitor's history section correctly refers to it as a bomb ship.
“Bomb ships or bomb ketches were made by removing the front masts of a warship to make room for one or more deck-mounted mortars. These ships could then bombard an enemy fortress from several miles away. The Star-Spangled Banner, the US national anthem, refers to such a bombardment.”