Here's some facts from the internet about this fortress:
Early in the Sixteenth Century, trade was established between Guatemala and Spain via what was then called the Golfo Dulce. Constant attacks by pirates in the Gulf of Mexico and incursions into Guatemala through the Rio Dulce made it necessary to defend the entrance to Lake Izabal where warehouses had been set up for goods entering from or leaving for Spain.
1595 The Governor informed King Philip II of Spain of the attacks suffered and it was decided to build a tower equipped with twelve artillery pieces and twelve soldiers. The tower was called the Sande Tower.
1604 After the first tower was destroyed it was rebuilt by Captain Don Pedro de Bustamante, from whom it took its name, the Bustamante Tower. It was around this time that the port at Santo Tomas de Castilla was founded.
1640 The pirates intensified their attacks in this area. Some rather famous (or infamous) pirates were involved in the attacks on the Rio Dulce including: Diego the Mulatto, Lieutenant of "Pegleg" Anthony Shirly. Shirly was a pirate of aristocratic birth, called the Adventurous Gentleman, the Highwayman from Jamaica and Puerto Rico. Also involved were Carefull and William Jackson who had their base of operations on the islands of Guanaja and Roatan. William Parker, known as the plunderer of Santo Domingo and Puerto Bello also made attacks on the Rio Dulce.