While procedures at the border are not complicated when entering by air (a de facto requirement for most travellers while the travel permit system remains in place), the necessary visa formalities before arrival can be quite time consuming.
Items of historic or cultural importance may not be taken out of Eritrea under any circumstances.
Most foreign nationals require visas from the Eritrean embassy or consulate accredited to their home country before they depart from that country.
While citizens of Uganda and Kenya may visit visa-free and citizens of Sudan can obtain visas on arrival, all other foreign nationals require a valid visa to enter Eritrea. In the majority of cases visitors are required to apply at the embassy or consulate that is accredited to their place of residence, a process than can often take months between application and approval, but increasingly travellers report that tour agencies in Asmara are able to provide visa-on-arrival invitations with minimal fuss. Whichever route you decide to pursue, begin the process well before your travels begin.