Where was the Texas Declaration of Independece written?

In a place called Washington on the Brazos.

Explanation:

The place is located along the Brazos River, which is an integral part of Washington County in the state of Texas. The place was founded even before Texas became an integral part of the United States. A declaration in that place was signed in March 1836. With the Declaration, Texas became an independent state that had previously been part of Mexico.