The anvil
Your rebound matters more than your arm.
The foundation
The anvil isn't a table you hit things on. It's the rebound — a good anvil throws your hammer back at you, so half the energy of every strike goes into the work for free.
What makes one good
face hard, flat, and smooth — no dings horn round, for drawing and curving hardie a square socket for bottom tools weight anvil should outmass your hammer 10:1
The test
Drop a ball bearing from 30cm. It should rebound nearly to your hand. If it thuds, the anvil is dead — soft, cracked, or a casting that looks the part and does none of the work.