Geographically the region where the three continents meet and connect to each other, from the line of Nantes - Lyon - Alps - Sava - Lower Danube, to the rock and sand desert of the Sahara. It can be divided into three parts:
1. The three peninsulas of Southern Europe (Iberia, Italia, Balkans)
Each means a separate front in itself, sharply separated from the body of Europe with mountains and rivers. There is no hinterland behind them for an attacker - just the seas and the resource thin Africa -, forces, war materials have to be transported from afar, nowhere to retreat, and if the separate fronts are established, they can't help out each other easily. While a defender will have Europe's resource rich areas behind them, short distances, and the central position means they can regroup and rebalance forces easily. Decisive action can be taken in the area of the Garonne or Ebro, the Po, and the Danube.
2. the Mediterranean sea itself with its islands and archipelagos
The shortest sea lane between the Atlantic and the Indian ocean for European powers. Three key areas are Gibraltar, Suez, and the narrow waters at Sicily and Tunis.
3. the shores of Northern Africa
A land bridge between the Atlantic and the Indian ocean. Its more developed pair runs in parallel south of the Sahara (I assume through the French colonies of Sahel to Djibouti), but that is outside of the European Strategic Space.