Cape Coast Castle – now a World Heritage Site – is one of about forty forts in Ghana where slaves from as far away as Burkina Faso and Niger were imprisoned. This former slave fortress could hold about 1,500 slaves at a time before they were loaded onto ships and sold into slavery in the New World in the Americas and the Caribbean.

At Cape Coast Castle, slaves were shackled and crammed in the dank dungeons. There was no space to lie down and no sanitation, with human waste littering the floor. Slaves could spend up to three months confined in these miserable conditions before being loaded onto ships.

Male captives who revolted or were deemed insubordinate ended up in the condemned cells – a pitch-black room where slaves were left to die in the oppressive heat without water, food or daylight. Rebellious women were beaten and chained to cannon balls in the courtyard.

These now silent cannons are a reminder of the military might of the British who used Cape Coast Castle to hold slaves for 140 years. It's estimated that between the late 17th and early 19th centuries, some three million West African slaves were shipped from here.

Fort Christiansborg, also known as Osu Castle, sits in the lively township of Osu in Ghana’s capital, Accra. It was built by the Danish, who originally traded in gold, then in slaves. The slave trade was so successful that they had to expand the castle to almost four times its original dimensions.

On the seaboard side of the coastal slave castles was 'the door of no return', a portal through which the captives were lowered into boats and taken to the slaving ships anchored further out at sea. Four in 10 did not survive the tortuous voyage. Those who made it across the Atlantic alive would never set foot in their homeland again.

Erected in 1784, the crumbling ruins of Fort Prinzenstein in Keta, east of the Volta River, are a reminder of Denmark's role in the transatlantic slave trade. As the first slave-trading country to abolish the practice in 1792, Denmark's barbaric involvement in shipping an estimated 120,000 slaves to the former West Indies is often glossed over.

It is easy to walk past this nondescript building in Ghana's capital Accra for anyone not aware of its historical significance. Built by the British as a trading post in 1673, Fort James was also used to hold slaves. It has a true history of chains – it was also used as a prison until 2008.

Built in 1482, Elmina Castle on Ghana's Cape coast is the earliest European structure erected in sub-Saharan Africa. Originally Portugese, it was later captured by the Dutch, who used it as a base for the Dutch slave trade with Brazil and the Caribbean. Under the flag of the Dutch West Indies Company, around 30,000 slaves a year passed through Elmina until 1814 when the Dutch abolished slavery.

50-year-old Abdul Sumud Shaibu shows a picture of his grandfather on his smartphone. "Look at his height," he says. "My ancestors were giants. Strong and well built." Some of his ancestors even fought against slave hunters, Shaibu says. Sometimes they won but not every time. Losing meant a lifetime of slavery.

Slave trade in West Africa flourished long before the first slaves were sold to the United States of America. By the end of the 15th century, Portuguese ships were bringing African slaves to the country. Occupied by the Europeans in the following decades and centuries, the African west coast turned into a hub for slave export.

The system was called triangular trade because major European powers brought goods such as weapons, textiles or alcohol to the West African coast, to be traded for slaves. From Africa the ships would sail to America, where the slaves were exchanged for coveted raw materials such as tea, coffee or cotton. These goods were then shipped back to Europe.

The living conditions during the journey across the Atlantic, the so-called Middle Passage, were inhumane. All the space available on the slave ships was used down to the last centimeter. The slaves were literally stacked on top of each other. They were chained and did not get enough food or water. Sick slaves were simply thrown overboard to keep infections from spreading.

In the "New World" the slaves were forced to work on cotton and sugar cane plantations. While the owners of the plantations were wealthy, the slaves remained poor. Living conditions for these slaves were harsh. Others worked in the cities as longshoremen or in the household of their owners. Many were used by the mining industry. Punishments and abuse were prevalent.

For many slaves violence was a part of their everyday life. They were whipped, had to wear shackles and were marked with branding irons. Their so-called "owner" could decide whether they were allowed to enter into a relationship. Slaves had no rights and could only hope to be released at some point. These shackles are exhibited today in a museum in the Ivory Coast.

The transatlantic slave trade reached its peak in the 18th century. West Africans accounted for about two thirds of African slaves brought to America. This map shows the coastal ports from which the ships set sail. It is not known how many Africans exactly were enslaved. Estimates point to 40 million people.

The Elector of Brandenburg, Friedrich Wilhelm, had the "Groß Friedrichsburg" colony built on today's Ghanaian coast. Ghana was called "Gold Coast" at the time of the slave trade. From there the Brandenburgers took part in the triangular trade, shipping about 30,000 slaves from 1685 on to the so-called "New World".

In Adidwan, a village in Ghana's Ashanti region, Nana Assenso visits the grave of his great-uncle Kwame Badu. His relative was named in memory of an ancestor who long ago was sold as a slave. Since then, the name has been passed on through the generations in the family. Nana Assenso's son is also called Kwame Badu.

An installation by Ghanaian artist Kwame Akoto Bamfo in the village of Nuhalenya Ada commemorates the enslaved ancestors. Although the British banned the slave trade in 1808 and this ban was confirmed by the Congress of Vienna in 1815, slavery wasn’t really abolished until around 1870. Today, the ruins of the former slave fortresses on Ghana's coast are a reminder of this era.

On April 4, 169 Gambian migrants returned home voluntarily. They had left The Gambia, in many cases more than a year ago, with the intention of reaching Europe. Their journey was cut short in Libya, where many were arrested and detained, often under dire conditions.

According to the International Organization for Migration (IOM), Gambians have reported that they were subjected to torture and lack of food while in detenion in Libya. Some have described the emergence of a "slave market," with captors demanding they call their families to ask for money in return for their release.

With the help of the IOM and the Gambian government, some Gambians were able to walk free from Libyan detention centers. The returnees arrived at the airport in Banjul, the capital of this small West African country, and queued to obtain emergency passports and a small amount of money to get them home.

Had they made it onto a trafficker's boat, as was their intention, they would have faced a perilous journey across the Mediterranean - most likely in an overcrowded rubber boat. More than 5,000 people are estimated to have drowned during such journeys in 2016.

In December 2016, long-term ruler Yahya Jammeh was ousted by opposition candidate Adama Barrow (pictured). Jammeh resisted, but eventually fled after West African troops threatened to remove him by force. This was seen as the beginning of The Gambia's transition to democracy. Peaceful, multi-party parliamentary elections followed in April 2017.

Gambian migrants who have made it to Europe are now facing the threat of deportation. Gambians currently make up the third-largest group of African refugees in Germany, but if The Gambia is designated a "country of safe origin," their applications for asylum might be rejected.

There are high hopes for the future of a democratic The Gambia. However, the country's authoritarian past - including reports of prison torture and the detention of opposition figures - still needs to be addressed. The EU has pledged 75 million euros to support Gambia's return to democracy and boost the economy.

Despite the fact that slavery is prohibited worldwide, modern forms of the sinister practice persist. More than 40 million people still toil in debt bondage in Asia, forced labor in the Gulf states, or as child workers in agriculture in Africa or Latin America.

Human trafficking is big business, according to the UN Office on Drugs and Crime. Sectors such as construction or mining exploit almost exclusively men, while victims of forced prostitution and exploitation in private households tend to be women. But something they all have in common: they are coerced into working through threats, the use of violence, or fraud. Most cases go unreported.

Because of poverty parents may sell their children as labor. That's often the case in Lake Volta, Ghana, where children are forced to work for fishermen. Parents are told their children will get to do an apprenticeship. But in reality, they're kept as slaves in appalling conditions. According to the International Labour Organization (ILO) there are around 10 million child slaves worldwide.

One in five girls is wed before she turns 18, according to the UN Children's Fund UNICEF. Not all of these marriages are forced arrangements. But every year millions of underage girls are forced into marriage before the age of 15. In many cases they're taken out of school and essentially live as unpaid laborers in their spouse's home. Many report physical and sexual abuse in the marriage.

All over the world, girls are exploited as house slaves — by their own family or by strangers. Poverty-stricken families may be promised that their children will get the chance to go to school. But once they're taken, these girls are locked inside the house and forced to work 12-14 hours a day. Many also suffer sexual abuse. The number of unreported cases is high — even in industrialized nations.

Under this form of slavery, victims are forced to work to pay off a debt. Often the debts continue to pile up, even if the whole family toils for 10 hours a day at the brickworks, or in their owner's quarries, fields or mines. Often the debts are also inherited by children. The ILO estimates there are around 30 million people working as debt slaves, most of them in India and Pakistan.

Illegal migrants are especially vulnerable to exploitation, regardless of where in the world they are. They often have nowhere to claim rights, usually cannot speak the local language, and don't know where they can turn for help. It's not clear how many illegal migrants work in agriculture in Europe alone. But many live in abysmal conditions as they slave away for well below the minimum wage.

The descendants of African slaves in Mauritania are called "Haratin." Although slavery is officially banned in the northwestern African country, people are still inherited or sold as property there. An estimated 600,000 women, men and children in Mauritania are currently exploited as domestic workers or in the agriculture sector. That's one-fifth of the population.