As Conspiracy enters the battlefield, choose a creature type. Creatures you control are the chosen type. The same is true for creature spells you control and creature cards you own that aren't on the battlefield.
It does not replace any use of creature types in card text.
This can grant a creature type to animated lands and artifacts that would otherwise have no creature type.
If you choose Wall, then your creatures can still attack because creature types don't confer abilities such as defender (any more than, say, choosing creature type Bird would confer flying).
"Legend" is no longer a creature type, and may not be chosen.
Affected spells, cards, and creatures lose all of their other creature types.
Replacement effects that modify creatures of a certain type as they enter the battlefield will apply (or not apply) after you apply this effect. For example, if Warrior is the chosen creature type and you control Bramblewood Paragon, a Runeclaw Bear would enter the battlefield with an additional +1/+1 counter.