I tried to turtle in Rome once, with Brutii, after playing Medieval. My plan was to build up Tarentum(?) and the other starting city, Croton?... It's amazing how much more difficult RTW is compared to MII, I was very close to losing that campaign and I had to fight several desperate battles to get out of the bind I was in after both greek city states and macedonia went to war with my 2 settlement "empire" :p I think I was at war with Gaul as well hehe.

Short answer to the OP, you turtle to use other units than spearmen and to make it a little bit harder for yourself i suppose.