Mortar

A mortar is a dedicated building and boat removal device.

Although it cannot harm living objects or other artillery, the humble mortar gets a massive attack bonus versus ships and an even bigger one versus buildings. Assumedly the attack prohibition was to stop people trading in their more expensive falconets for cheaper and more-effective-building-smashing mortars, making the falconet's role (early field support AND building demolition) redundant.

Mortars can get card upgrades to make them more appealing, the Dutch Coehorn Mortars (Team card: 20% damage bonus) and the Russians can field the much-loved Unicorne Mortars (Player card: gold cost -50% [175g], 20% damage and training time -50%).

Tactics
Mortars are used for two things - obliterating buildings and ships. Due to the damage bonus (10x), unupgraded mortars hand out a neat 600 damage per shell at whatever poor building is in the way (before any building resistances), allowing a couple of them to level a fort within a few salvoes. Their weakness lies mainly in their inability to fight units, the slow speed prerequisite of all artillery pieces, and awfully slow setup times, allowing people to send out cavalry, infantry, even Villagers to wipe it out. (you try and pike up 1/4-1/2 a tonne of iron with 3 other people and you'd be fairly slow too)