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A model “polymer chain in an array of obstacles”, which is more detailed and convenient variant of the generally accepted model “chain in a tube”, is proposed to be useful in molecular theories of systems with strongly entangled polymer chains. It is shown that the problem of the behaviour of the chain in the array of obstacles can be reduced to the problem of random walks on a tree (in the discrete realization) or on a Lobachevsky-Riemann space (in the continuous realization). The properties of the closed polymer chain in the array of obstacles are considered. The analogy between this problem and the problem of the statistics of lattice animals is established and it is shown that a long, closed, unentangled polymer chain in the array of obstacles takes a strongly collapsed conformation.