There are two ways to reach this cache.
The rocky way: Go through the tunnels to the beach and turn right as soon as the rockwall to your right allows it (WP R1). Follow the path to a short climbing part and ascent the big rock cleft via old stairs.
This used to be the official way to reach Sa Caldobra before they built the tunnels. Go up into the col, descent a few meters and turn left to reach the beginning of a short via ferrata called the Pas de s'Aranya (WP R2). A thin iron rope helps you ascent a 20m wall (Ferrata grade B-C). Now follow the stone cairns to the tower. This is easier said than done, since the trail is almost invisible. (Terrain rating: 4.5*)
The green way: Start at the big parking place and find a gate (WP G1) in its western loop. Go though the gate and follow the fence to the left for a short while. You should soon find the overgrown remains of an old path (many cairns) which leads you to a col to the south of the tower (WP G2). Follow the stone cairns to the tower.
Good shoes and excellent pathfinding abilities are needed for this cache. If you take the rocky way you also need some climbing abilities. You can find pictures and a track for this trip here.
The tower has been built in 1596 and was one of 85 similar towers built to warn the populace of approaching pirates - more infos on this page. Good maps of the area are published by Editorial Alpina