The Colombian Army has a year round presence at La Ciudad Perdida. This is to maintain the security of the ruins. Most of the time though you will find soldiers posing for tourist photos or selling their surplus gear.
The region was formerly a hot spot for cocaine production and guerrillas but with the increased army presence in the area and security gains across the country, the risk is extremely low for tourists. The first and only kidnapping occurred in 2003. It ultimately ended with the safe release of the victims. The Colombian government has mandated the protection of tourists as part of its attempt to boost its tourism industry.
The army presence at the ruins mostly consists of soldiers fresh out of training. Don’t be afraid when you see them heavily armed. They’ll be happy to pose for photos or sell you authentic army supplies.
If you’re interested in buying a souvenir from them, such as a bandanna or backpack feel free to ask. The general consensus from previous travellers is that the prices they offer are fixed. But you may still be able to negotiate by talking to a few of them.
The soldiers are stationed at the Lost City for long periods of time. They’re flown in and out by helicopter. In rare instances, you may be able to negotiate a flight out.
Travellers have also been able to charge batteries at the army camp near the ruins. If you happen to run out of power while you’re exploring the Lost City then it might be worth checking to see if your guide can ask the army for a favor.