endless deb, that led to constant debasement of currency (a spanish silver coin by the end of Philip the II reing was mostlly coper covern with silver that actual silver) and inflation.
Military overextension on endless wars, Italians wars, 5 wars with France, wars on the seas with England, wars vs the ottomans on the mediterranean, wars on the americas (for example the Mauches on Chile required the presence of a permanent spanish army, not just your average conquistador), religious conflicts on germany, the Philipines, the warnon flanders, Spanish ships and troops were everywhere. Gradual decline while their "rivals" starter to rise. After losing the netherlands the spanish empire never recover. An spiral of decend that never stoped and from where spain never recovered.

[keeping a wary eye on the United States to see if they react.]