From Populous Wiki
Sidedooring or Sidedoor is a technique used very commonly in the Populous Multiplayer. Although it is not used by the computer opponents in the Single Player version of the game the player can choose to Sidedoor the computer opponent if they want to.
It is made to attack and catch the enemy by surprise. It is a very common strategy used among the more experienced players of the Populous Multiplayer. It is constructed by making Landbridges or Flatten to connect some part of the opponents base. Usually a Shaman is used to create or defend and attack through a Sidedoor. Another sidedoor mostly uncommon is by using an alternate route form the map to reach the opponent's base while keeping them busy.
If successful the Sidedoor could become a critical factor in the game.
The most effective way to defend a Sidedoor from an opponent can be achieved with a Shaman and a few Firewarriors and Warriors. Sidedoors can also be defended by placing Firewarriors in Guard Towers or making them guard around the side that could be attacked, the same way a player would defend any other entrance.
Another strategy is for the defending player to initiate a strong attack against the attacker. Attacking from the side will typically leave a front attack vector not defended by a shaman, meaning that it may be the ideal time to launch a strong attack (possibly Doubling).
It's also worth noting that a sidedoor is a two way path, so it's usually possible for someone who has been sidedoored to reverse the attack and end up at the heart of the attacker's base without having to charge the spells needed to get there.
Is Sidedooring Allowed?
Unless agreed upon by all players before the game, yes. It can turn the tables of a fight, by invading an undefended entrance to an enemy base. Because of its ability to reverse the direction of the game so quickly, many players will complain or try to make new rules blocking the use of a side door. However, it is a valid strategy that some levels (such as Face Off) are designed to include, so unless all the players have agreed against it, then nothing has been done wrong.