At a glance:

Vastu Purusha is the diety responsible for the strength and happiness in the site.
Three positions for the the Vastu Purusha are mentioned.
These positions indicate the various seasonal changes an the permanence of the structure

Brihat Samhita describes the Vastu Purusha as follows:
It is said some Being obstructed the earth and the sky with its body. The gods suddenly caught the being and laid it face down on the ground. Whichever limbs were held by different gods had those very gods as their presiding dieties. The creator ordered that the Being be the god of the site (or house).

The story also runs as follows:
When Lord Shiva is fighting the. demon called Andhaka, his (the Lord s) perspiration falls to the ground. Out of this is born the Vastu Purusha. Being hungry he starts devouring everything that comes his way. The gods then go to Lord Brahma. He tells them to hold the being face down. 45 deities press him down. Lord Brahma blesses him saying that he will be the deity of all plots and offerings have to be done to him. In return Vastu Purusha is said to take care of the inmates of the house.

The above figure names the dieties responsible to keep the Vastu Purusha on the plot.

The Vastu Purusha is said to have three positions
viz.,
l)Nitya Vastu
2) Chara Vastu and
3) Sthira Vastu.

Nitya Vastu

The Vastu Purusha’s gaze changes every three hours in a day.

Chara Vastu

The Vastu Purusha s gaze is directed towards South during Bhadrapada, Ashwayuja and Karthika corresponding to September, October, November and December months; towards West during Margasira, Pushya and Magha corresponding to December, January, February and March months; towards North during Phalguna, Chaitra and Vaishaka corresponding to March, April, May and June months and towards East during Jyeshta, Ashada and Shravana corresponding to June, July, August and September months.

It will be seen that the Vastu Purusha’s change of position are closely linked to the various seasonal changes.

Sthira Vastu

The third and most important position is the fixed position of the Vastu Purusha where his position is as shown on next page.

Here his feet are on the South-west, his left arm on the North-west, his head on the North-east and his right arm on the South-east.

Sthira in Sanskrit means fixed, stable or permanent. Sthira Vastu is therefore concerned with the construction of a house, temple, village, town, etc and their permanence.The importance of the shape of a building or a site and scientific aspect of the Vastu Purusha who lies in a perfect square with his head and seat on the SW-NE diagonal and hands on the SE-NW diagonal are highlighted in Brihat Samhita with a stanza which means:
If the “Vastu Nara” god

1. Has no right arm, the owner will lose wealth and will be miserable through women.
2. If he is without the left arm, there is loss of money and food.
3. If his head is absent, he will fall from all virtues.
4. If his feet are missing male children will die, the master becomes weak and there will be troubles through women.
5. On the other hand if the House god is endowed with all the limbs in fine shape, the inmates of the house will be happy and live with wealth and honour.

It is also clear from the stanza that irregular shapes are not recommended.