Attacking is a style of play in which the player attempts to accumulate as much land as possible by invading dominions in other realms from the start of the round. The player will train both offensive and defensive troops, trying to defend the competing attackers while building up offensive power to slow down and threaten other players.
There are several ways in which a player can choose to attack from out of protection (OOP) in OpenDominion. Success at attacking depends on the desired pace of invading, racial selection, and the attacker's building composition (referred to as a player's "build").
Pace of Invading
There are two distinct and common methods of attacking from out of protection: fast attacking and slow attacking.
Fast attackers seek to gain land quickly, invading as often as possible. Fast attacking is suited to races with population racial bonuses and typically train cheap, inefficient troops. When properly executed, this strategy enables fast attackers to gain an early-round advantage and potentially win the round with that edge. However, the cost of running inefficient troops comes later in the round when other players have had time to catch up. A fast attacker who fails to secure a significant early-round advantage is not able to compete for the duration of the round.
Slow attackers tend to invade at a safer, slower pace. Instead of training cheap and inefficient troops, a slow attacker will train almost exclusively elites and build investment buildings such as diamond mines to attain high improvements. Slow attacking is suited to races with more expensive, efficient troops. This risk-averse strategy can consistently grow throughout the round, hoping to eventually overtake fast attackers at some point later in the round.
Not every race is equally suited to attacking from out of protection, but every race is capable of attacking to some degree. Certain races also lend themselves to a particular build, speed of invading, and overall strategy.
For slow attacking, any race can be used that has both efficient elite defensive and offensive troops, though Gnome is difficult to attack with early in the round due to high ore troop costs.
An attacker's build depends heavily on the chosen race and speed of attacking, but most strategies will require some similarity in terms of needing Farms, Homes/Barracks, Lumberyards, etc. Almost all attackers will build maximum Smithies to reduce military costs.
Fast attackers are more likely to use mod buildings (Gryphon Nests and Guard Towers), run higher Alchemies early in the round, and have a higher percentage of Homes/Barracks to keep their population growing as large and as quickly as possible.
Slow attackers are more likely to maintain high improvements, building Diamond Mines, Ore Mines, or Lumberyards to invest with depending on the chosen race.