Machida is an interesting city where old and new are mixed together.
Literal
Machida [topic-は] old things [and-と] new things [subject-が] have-mingled interesting city is.
入り交じる (to intermingle) is a compound verb from 入る + 交じる, used in its past form as a modifier for 街. もの after adjectives nominalizes the quality — 古いもの (old things), 新しいもの (new things). The が inside the relative clause marks what does the mixing, while は outside marks Machida as the overall topic.