Is it easier to learn L2 that is similar to L1?

Literature review:
According to my case study, I have reviewed several types of research that were conducted to find out the importance of L1 in learning a second language, the differences, and similarities between two languages, and what factors influence efficient second language acquisition. 
Contrastive Analysis, a systematic study that was implemented by many researchers from the 1940s to the 1960s to compare two languages: the native and the target languages. Charles Fries (1945) was the scholar who developed the idea of contrastive analysis and defined it as a fundamental component of the methodology of second language teaching (Alkhresheh, 2013). According to CA, the best way to identify the effectiveness of L2 acquisition is to implement a comparative analysis of L1 and L2. Results of the analysis can demonstrate that the traits that are similar to the NL of a learner will advocate the easy learning process while those that are different will cause difficulties in L2 acquisition. When the structures of the TL are different from the NL, the learner can encounter problems in acquiring L2. Success in learning can be reached when the learner can overcome these challenges. However, when the structures of L1 and L2 are identical, the learning process can be relatively easy for the learners.
According to Lado (1957), CAH proposes that challenges in acquiring a new (second) language are originated from the differences between the target language and the native language of a language learner. Learners tend to transfer forms and meaning from their native language to the target language. Not all difficulties and differences in L2 acquisition are equal. They have degrees of easiness or difficulties. When two languages are identical a positive transfer occurs and if there are differences between them negative transfer, or interference is resulted.
Errors that are made by L2 learners are predicted from interference by the first language. In this regard, such a phenomenon is known as a negative transfer. While analyzing errors made by L2 learners, researchers such as Corder (1967) and Richards (1971) claim that negative transfer is one type of interlanguage error that is not specific to the native language. 
Well-known SLA linguists such as Stockwell, Bowen, and Marlin (1965) developed a model of prediction that is called a hierarchy of difficulties.
“Level 0—Transfer. No difference or contrast.