Fortifications are a structure in Sim Mode, introduced in Actraiser Renaissance. They provide defense during Settlement Raids, and careful placement (or re-deployment) of Fortifications is essential to survive a raid. Each Fortification has a level ranking, which raises along with a realm's Civilization Level, though the fortification must be manually upgraded at the cost of Resources. Each realm will develop the ability to build fortifications independently. It's best to have a variety of fortifications, as virtually all raids will have enemy units that require at least two types of fortifications to deal with.
It's also important to consider the resident Hero of a realm, as they often have properties that mimic a type of Fortification. Philotas, for example, functions much like a mobile Gatehouse, so building other Fortification types may be a better option in Fillmore. Also, each hero provides a unique, passive bonus to fortifications. Shemaal, for example, is best placed around damaged fortifications for his passive regeneration.
Gatehouses are placed on main roads and provide a physical block to all enemy forces that meet it. It deals physical melee damage to all enemy units on regular intervals. It's generally a good idea to place these on direct routes to the main temple. They don't provide any protection against flying enemies, which can attack Gatehouses without reprisal.
Blockhouses are archer towers, dealing physical ranged damage to a single nearby target. A long string of enemies can present problems by overwhelming a tower, due their one-target-at-a-time effect. A good amount of flying enemies encountered in raids will have magic defense, so Blockhouses remain critical even in later areas.
Magehouses deal magical ranged damage to a small area by launching a fireball at enemies that enter its range. While very strong against melee ground forces, it's less effective against flying forces and virtually useless against units with magic defense (with the blue hexagonal shield). These are the last fortifications to be developed and while it may be tempting to focus on building Magehouses, the other fortifications are still essential to base defense.