Of course, when you invest in lower credit rated investments, you take on credit risk — the other risk in debt instruments,
apart from interest rate risk.
Although you are correct that treasury bonds are safe from default due to their being backed by the U.S. treasury, they are not
immune from interest rate risk.
Price risk is the risk that the fair value or future cash flows of a financial instrument will fluctuate due to a change in market prices (other than those
arising from interest rate risk or currency risk), whether those changes are caused by factors specific to the individual financial instrument or its issuer, or factors affecting similar financial instruments traded in the market.
How tall a ladder you build depends on how much money you have, how insulated you want to be
from interest rate risk and how much easily available cash you want to have.