Note: You MUST have an Poseidon Temple in your city before you may purchase a Poseidon House.
Strategy
Offensive
It can take a long time to destroy a building surrounded with Poseidon houses. Destroy them before attempting to destroy the building. Ares' Whirlwind Attack, Zeus' Lightning Storm or Artemis' Multi-Shot are good abilities for this.
Once the shield has been destroyed, the house is ineffective.
Destroying the Poseidon Temple will destroy all Poseidon houses. If possible, destroy the Temple as soon as you can.
Defensive
This house is the only one that only affects a single target. Scatter them around your base and make sure your Poseidon Temple is protected.