Though the Vedas insist that the reality is a Principle, it allows worship of personal God,Ishwara.
For Realization, the contribution of the Gd is necessary.
Even for the Karma to take effect God is necessary,Adi Shankaracharya avers.
This personal God differs from individuals to individuals, depending on their dispositions and their tendency towards a particular trait or emotion.
So you can worship God as Male, Female or an impersonal image, Linga.
It could be worshiped as one would a Child, Father, Mother,Friend, Love or even as a Foe!
Later came the Agamas, where collective worship was stressed(which is not in the Vedas).
The Agamas laid down Rules for Construction of Temples, sanctifying them with Mantras and the daily Rituals to be followed.
Some great Temples were built and they are in existence even today with Pooja being offered as laid sown by the Agamas.