Final answer:
Sparta fortified Decelea to control a key military position, disrupting Athens' economy and supply routes, and using their land warfare superiority to maintain pressure on Athens.
Step-by-step explanation:
The Spartans fortified Decelea during the Peloponnesian War to control a strategic military position. This move was a calculated strategy to weaken Athens. By establishing a permanent base in Decelea, located in the Athenian territory, Sparta was able to disrupt Athenian logistics, raid their lands, and effectively maintain pressure on their economic resources which were already strained by years of conflict.
Furthermore, as the Spartan army was superior in land warfare, controlling Decelea allowed them to continuously threaten Athens, which depended on its navy for supplies, rather than land routes. Sparta's decision to fortify Decelea also demonstrated a shift from their traditional land-based dominance towards combined land and naval warfare capabilities, which ultimately helped them to prevail in the war.